camera.device@3.2: Clarify documentation

- Clarify handling of input/output buffer handles, and
  that they must be null in CaptureResults.
- Clarify handling of fence handles and when they may/must be null.

Test: Compiles; comment-only changes
Bug: 32991603
Change-Id: I70367aa490fb0027791e6e35bb34c54601a0d144