Update API docs for broadcasts.

ACTION_NEW_OUTGOING_CALL is no longer sent as an ordered broadcast, so
update API docs to clarify this.  The full replacement has been available
since Q, so this change is a first step to finally removing the broadcast
entirely.

Test: Docs only change
Bug: 224550864
Change-Id: I552e8f616805768089a692eb8d23477cfd5090e3