Add a note about "read-only" to combobox pattern

[carmacleod]

Background discussion in #1026 (comment).

Some possible words:

NOTE
Additional authoring considerations for using aria-readonly and aria-disabled:

• Limit use of aria-readonly to combobox widgets that have a text field. A read-only combobox is in the tab sequence and its value can be read, however its popup cannot be opened, and its value cannot be changed.
• If a select-only combobox widget (i.e. a combobox without a text field) is not operable, use aria-disabled="true" to convey the disabled state to assisstive technologies.

The note could go right before the examples. @mcking65 please review.
Question: Should we say anything about a disabled read-only combobox? :)

[JAWS-test]

Should we say anything about a disabled read-only combobox?

A disabled read-only combobox should be the same as a simple disabled combobox. When I use disabled and read-only on an input element, read-only is ignored (see for example in Chrome, F12, Tab "Accessibility")