camera2: Add some more docs to getConcurrentCameraIds

- specifying the order of camera open and session configuration.
- explaining the use of CONTROL_ZOOM_RATIO with concurrent operation of cameras.

Bug: 151891611

Test: make doc-comment-check-docs

Change-Id: I3ee3f114f7295570aa6af5dbe35bb32db555811b
Signed-off-by: Jayant Chowdhary <jchowdhary@google.com>