Camera: Clarify doc for registerAvailabilityCallback

Camera availability callbacks's order matter. If the application runs
the callbacks on an executor, and the executor is implemented using
multiple threads, the order of the callbacks may change.

Test: Build and read docs
Bug: 263235259
Change-Id: I88913215e0545a096958aaba6b7b2b7d73a4f0a0
1 file changed