Update InCallService API documentation to clarify role and binding req.

Update InCallService API docs to clarify what happens if an app no longer
fulfils the requirement of the dialer role.  Also update documentation to
clearly indicate when the platform may show a warning to the user due to
a null binding.

Test: Docs only change.
Fixes: 236864749
Change-Id: Ief6225513a7aefefa9c029a02cc1a3511b236d44