Docs: Add some details to the User Defined Redirects (#7894)

- Move the "Implementation" section further up as a note, so that it's
  more visible (I missed it myself and spent some time trying to test
  redirects on 200 which do not work). Remove part about not running
  logic as it's no longer accurate.
- Add mentions to the use case of setting Exact Redirects to have
  Page Redirects but applied only to a specific version. It's common
  to move pages only in the `latest` branch/future stable branches,
  without doing the change in older branches.
- The button to confirm a redirect is now named "Add".

docs/user-defined-redirects.rst

@@ -12,10 +12,15 @@ Quick Summary
 * From the left navigation menu, select :guilabel:`Redirects`.
 * In the form box "Redirect Type" select the type of redirect you want. See below for detail.
 * Depending on the redirect type you select, enter FROM and/or TO URL as needed.
-* When finished, click the SUBMIT Button.
+* When finished, click the :guilabel:`Add` button.
 
 Your redirects will be effective immediately.
 
+.. note::
+
+    For the time being, redirects are only implemented in case of a
+    *404 File Not Found* error.
+
 Redirect Types
 --------------
 
@@ -68,8 +73,10 @@ You would set the following configuration::
     From URL: /example.html
     To URL: /examples/intro.html
 
-Note that the ``/`` at the start doesn't count the ``/en/latest``,
-but just the user-controlled section of the URL.
+Note that the ``/`` at the start doesn't count the ``/$lang/$version`` prefix (e.g.
+``/en/latest``), but just the user-controlled section of the URL.
+If you want to set directs only for some languages or some versions, you should use
+:ref:`user-defined-redirects:exact redirects` with the fully-specified path.
 
 .. tip::
 
@@ -117,6 +124,9 @@ The readers of your documentation will now be redirected as::
   docs.example.com/en/2.0/dev/install.html ->
   docs.example.com/en/3.0/dev/install.html
 
+Similarly, if you maintain several branches of your documentation (e.g. ``3.0`` and
+``latest``) and decide to move pages in ``latest`` but not the older branches, you can use
+*Exact Redirects* to do so.
 
 .. tip::
 
@@ -132,13 +142,3 @@ If you switch between *HTMLDir* and *HTML*, your URL's will change.
 A page at ``/en/latest/install.html`` will be served at ``/en/latest/install/``,
 or vice versa.
 The built in redirects for this will handle redirecting users appropriately.
-
-Implementation
---------------
-
-Since we serve documentation in a highly available way,
-we do not run any logic when we're serving documentation.
-This means that redirects will only happen in the case of a *404 File Not Found*.
-
-In the future we might implement redirect logic in Javascript,
-but this first version is only implemented in the 404 handlers.