Make minor documentation improvements

- Point to ExploreByTouchHelper in A11yNodeProvider
- Point to appropriate properties and methods from events
- Give example of click dispatch of non-screen-reader services, as a
  fallback to CLICK failing

Test: n/a
Bug: 289400219
Bug: 287276093
Change-Id: I8e86133745f8517ca41df801414c4cc04200ebe7