Camera: Clarify SCALER_CROP_REGION usage

1. Document same SCALER_CROP_REGION may map to different aspect
ratio/output size for different sessions.
2. Remove the text in ZOOM_RATIO about using SCALER_CROP to change
aspect ratio.

Test: Build and read docs
Bug: 168493526
Change-Id: I405fe52d0e0a14e6a1cfbec169d33f20715fa4fb