Add docs for the iOS and Android select component

UNRELEASED.md

@@ -7,3 +7,4 @@ Use [the changelog guidelines](https://git.io/polaris-changelog-guidelines) to f
 ### Documentation
 
 - Added iOS and Android examples for the TextField component ([#1893](https://github.com/Shopify/polaris-react/pull/1893))
+- Added documentation for the iOS and Android select component ([#2041](https://github.com/Shopify/polaris-react/pull/2041))

src/components/Select/README.md

@@ -1,6 +1,10 @@
 ---
 name: Select
 category: Forms
+platforms:
+  - android
+  - ios
+  - web
 keywords:
   - on off
   - switch
@@ -19,42 +23,37 @@ keywords:
   - long list of options
   - long option list
   - separate error message
+  - ios
+  - android
 ---
 
 # Select
 
-Select lets merchants choose one option from a list in a dropdown menu. It
-works well for lists of more than four choices when displaying them could
-clutter up the interface.
+Select lets merchants choose one option from an options menu. Consider select when you have 4 or more options, to avoid cluttering the interface.
 
 ---
 
 ## Best Practices
 
 The select component should:
 
-- Be used for lists of four or more items
-- List items within the menu alphabetically or some other logical order so
-  merchants can easily find the selection they need
-- Provide a label to clearly identify the content being presented in the drop
-  down menu
-- Have a default option selected, where possible
-- Have a placeholder option with the text “Select” if there is no logical
-  default option
+- Be used for selecting between 4 or more pre-defined options
+- Have a default option selected whenever possible
+- Use “Select” as a placeholder option only if there’s no logical default option
 
 ---
 
 ## Content guidelines
 
-### Field label
+### Select label
 
-A label is a short description of the requested input. Labels are not help
-text, and they don’t provide instruction, but they should be meaningful and
-clearly indicate what is expected. Labels should be:
+Labels should:
 
-- Placed above or beside the form field
-- Short and succinct (1–3 words)
-- Written in sentence case (the first word capitalized, the rest lowercase)
+- Give a short description (1–3 words) of the requested input.
+- Be written in sentence case (the first word capitalized, the rest lowercase).
+- Avoid punctuation and articles (“the”, “an”, “a”).
+- Be independent sentences. To support [internationalization](/guides/internationalization), they should not act as the first part of a sentence that is finished by the component’s options.
+- Be descriptive, not instructional. If the selection needs more explanation, use help text below the field.
 
 <!-- usagelist -->
 
@@ -80,25 +79,24 @@ clearly indicate what is expected. Labels should be:
 
 <!-- end -->
 
-### Menu options
+### Select options
 
-The list of options in a menu should:
+Options should:
 
-- Be concise but still give the merchant enough information so they can easily
-  make a selection
-- Be arranged alphabetically or in some other clear logical order
-
-### Placeholder option
-
-The placeholder option should be the text “Select”.
+- Use “Select” as the text for a placeholder option, if present
+- Be listed alphabetically or in another logical order so merchants can easily find the option they need
+- Be written in sentence case (the first word capitalized, the rest lowercase) and avoid using commas or semicolons at the end of each option
+- Be clearly labelled based on what the option will do
 
 ---
 
 ## Examples
 
 ### Default select
 
-Use when a merchant needs to choose one option from a list of four or more.
+<!-- content-for: web -->
+
+Presents a classic dropdown menu or equivalent picker as determined by the merchant’s browser.
 
 ```jsx
 class SelectExample extends React.Component {
@@ -129,8 +127,28 @@ class SelectExample extends React.Component {
 }
 ```
 
+<!-- /content-for -->
+
+<!-- content-for: ios -->
+
+The iOS picker expands in-line. Merchants scroll to select the item they want.
+
+![iOS select, and select with option menu](components/Select/ios/default.png)
+
+<!-- /content-for -->
+
+<!-- content-for: android -->
+
+The Android menu is similar in behavior to the web dropdown.
+
+![Android select, and select with option menu](components/Select/android/default.png)
+
+<!-- /content-for -->
+
 ### Select with inline label
 
+<!-- example-for: web -->
+
 Use only for cases where the select must fit on a single line, such as in a toolbar.
 
 ```jsx
@@ -182,13 +200,23 @@ Use for selections that aren’t currently available. The surrounding interface
 />
 ```
 
+<!-- content-for: ios -->
+
+![Disabled select component on iOS](components/Select/ios/disabled.png)
+
+<!-- /content-for -->
+
+<!-- content-for: android -->
+
+![Disabled select component on Android](components/Select/android/disabled.png)
+
 <!-- /content-for -->
 
 ### Select with validation error
 
 <!-- example-for: web -->
 
-Use to let merchants know if their input is valid or if there’s an error. Whenever possible, validate input as soon as a merchant has finished interacting with a field (but not before). If a field already has an error, validate and remove errors as the merchant types so they can immediately see when an error has been fixed.
+Use to let merchants know if there’s a problem with their selection. For selects, a selection is typically invalid only when using a placeholder option (“Select”) and no other selection has been made.
 
 ```jsx
 class ValidationErrorExample extends React.Component {
@@ -216,6 +244,8 @@ class ValidationErrorExample extends React.Component {
 
 ### Select with separate validation error
 
+<!-- example-for: web -->
+
 Use to let merchants know when their select input is invalid in the context of a group of form inputs that the select depends on.
 
 When the `error` prop has a boolean value of `true`, the select component indicates to merchants that their input is invalid without rendering an error message directly below it. It anticipates that an inline error component exists separately within the form.
@@ -299,8 +329,5 @@ class SeparateValidationErrorExample extends React.Component {
 
 ## Related components
 
-- To let merchants make a single selection from a list with four or fewer
-  options, [use the choice list component](/components/forms/choice-list)
-- To present merchants with a list of choices where they can make multiple
-  selections, [use the choice list component](/components/forms/choice-list) with
-  the `allow multiple` option
+- To let merchants select one option from a list with less than 4 options, use [the choice list component](/components/forms/choice-list)
+- To create a select where merchants can make multiple selections, or to allow advanced formatting of option text, use an [option list](/components/lists-and-tables/option-list) inside a [popover](/components/overlays/popover)