camera2: Documentation updates for ultra high resolution sensors.

Bug: 195987968

1) Document mininum pixel array size restriction for ultra high resolution sensors.
2) Add docs explaining mandatory stream combinations for ultra high resolution sensors.

Change-Id: Ic630259a57906c475210465588eda1a5cc3f68ec
Signed-off-by: Jayant Chowdhary <jchowdhary@google.com>