update docs for Connection#onHold()

The topic adds a transactional way to verify call state changes.
Connection#onHold can now lead to the call being disconnected if the
Connection#setOnHold is not called within the 2 second timeframe. The
docs should be updated to reflect this behavior change.

Test: build + existing Telecom test suite
Fixes: b/267234374
Change-Id: I910843d4e8adec8d38cd32915f223540cefb9ac6