update docs on moving VoIP calls out of starting states before 60 sec

VoIP calls must now move out of the starting state before 60 seconds or
else they will be disconnected.

Bug: 360298368
Flag: com.android.server.telecom.flags.disconnect_self_managed_stuck_startup_calls
Test: manual
Change-Id: Iace0625c40f51e91d9e8d43dca22b0b22f76ee6d