Camera: Update doc for stale queued buffer in input surface

In case a reprocessable capture session is re-created, any pending
queued buffers in the input surface will be flushed. This makes sure
newly created reprocessable capture session won't be stuck waiting for
stale buffers from previous session.

Test: Read docs
Bug: 138729150
Change-Id: Ia14221f31a6d4f6ebf35f630036ece66de1d0874