ICameraProvider@2.6: Add documentation about resource cost constraints for...

ICameraProvider@2.6: Add documentation about resource cost constraints for concurrent camera combinations.

- Also clarify requirements for MONOCHROME devices supporting Y8 outputs.

Bug: 150225538

Test: builds (doc change only)

Change-Id: I992f1bcee4bf266b6e66ddb32ece8291381c8e56
Signed-off-by: Jayant Chowdhary <jchowdhary@google.com>