Update MediaProjection documentation to mention onStop() reasons

Clearly call out that MediaProjection#Callback#onStop() can be called due to the system and/or the user stopping MediaProjection due to various reasons.

Bug: 352133031
Change-Id: Ifc1c8be955cb7bd46c5aa71335424799c1a5b282
Test: n/a
Flag: DOCS_ONLY
2 files changed