camera2: Add documentation for getConcurrentStreamingCameraIds specifying multi-process use.

Add documentation specifying that camera devices advertised by
getConcurrentStreamingCameraIds() refer to those devices which can be
configured concurrently by the same client application.

Bug: 150225538

Test: m offline-sdk-docs

Change-Id: I4997f10ca73de45fd58d22261c7456402d761a57
Signed-off-by: Jayant Chowdhary <jchowdhary@google.com>