Update RemoteInput#setChoices documentation

The behavior that #setChoices didn't render on mobile devices from apps
that target SDK < P went undocumented. Because this is changing in P and
above, this documentation needs to point out this behavior.

Also updated formatting and style of docs to improve interop with
kotlin.

Test: none
Bug: 79244189
Change-Id: Iabfe3b2ddcc369e5ec15f9773481468cbc48dabb