Clarify REMOTE disconnect cause API docs.

Update API docs for REMOTE to make it clear that this disconnect cause is
used both for user-initiated disconnection on behalf of the remote party,
and for cases where the remote party never answered the call.

Test: Docs only change
Fixes: 280505750
Change-Id: Id718c75e2b748d2b5d5778f89d3dd7a420845418