Docs: Note that a Surface acts like a weak reference

This causes frequent programming errors, when developers assume
that holding onto a Surface will keep its associated SurfaceTexture,
ImageReader, etc, also alive.

Bug: 31551063
Test: m offline-sdk-docs, manual viewing of result
Change-Id: I5fb5bb3e3c80c7d5d735417b1697e0fe9a62fc46