Add thread safety documentation

An upcoming change will remove "synchronized" from the API docs. This change
documents those cases where the guarantees can be determined from code
inspection.

Bug: 25767152
(cherry-picked from commit bf0dc0fba790cf95f76870c37469703f8f20a57c)

Change-Id: I328b96328e89950b90d537bf0a6a704242de4993