Clarify API docs for getCannedTextResponses.

The API docs do not clearly indicate the fact that canned text responses
may not be available when a call is first added.  Updating the API docs
to clarify this and reference the listener a developer can use to be
informed of changes to the canned responses.

Test: Docs only change.
Bug: 162843384
Change-Id: I52ced96439b2dfa31c1f2428979ab54466763afa