Clarify documentation for notification a11y events

This javadoc was confusing the TalkBack team, so I've
clarified that these events can come from either a
notification or a toast. I also removed a note that I
found confusing and, as far as I could tell, was not
correct anyway.

Bug: 34851364
Test: Docs changes only. Relying on TreeHugger.
Change-Id: Ie5c6efd6e32ce1e33f8cc45c795cda7d7b2e6583