Update Javadoc for `TelecomManager.endCall()`.

Clarify that a ringing call is only rejected if it is the only call. Otherwise, the foreground (active) call is ended. Also, extend the privileged caller requirement for ending calls to include transactional calls.

Flag: EXEMPT docs only change.
Test: None; this change updates docs to match the actual API behavior.
Bug: 424787945
Change-Id: I13976dd8f81b786bf070b5b84b93665799abfe7a