Camera: Document limitation on private reprocessing

For private reprocessing, the ImageReader used to produce images into
the ImageWriter isn't aware of the overridden format. If the HAL decides
to override the input IMPLEMENTATION_DEFINED format, reprocessing will
fail.

Document this limitation in the HAL interface.

Test: Build
Bug: 266954992
Change-Id: I9bd05e9682e4d11870bf9bd25763f28291faef75