Add clarification text on getCallerNumberVerificationStatus docs.

Update API docs to make it clear what the verification status means and how
it can be used by CallScreeningService and CallRedirecitonService
implementations.

Test: Build API docs and view offline.
Bug: 150343312
Change-Id: I56f92f5e3ab544e31337a8147134baac33bb4ea9
1 file changed