Clarify accessibility event documentation

Removing suggestions to get information from events that
can instead be extracted from the node hierarchy.

Clarifying window change and announcement.

Comment changes only. I'm trying to steer developers to
use the APIs as we currently thing about them, but I'm
not changing any behavior.

Test: make
Change-Id: Ieecedbb98bdf11cf0cf4a29856db497986faaca6