Update docblocks for methods potentially returning null

Update docblocks for methods potentially returning null

These were discovered by setting `null_casts_as_any_type` to true in
phan, and filtering by `PhanTypeMismatchReturnNullable`. Of course there
are others, some of which are false positives, but we cannot suppress
them now (or the UnusedSuppressionPlugin will complain).

Change-Id: Ia8443e575c22f47a6d8c63038f4e7ac36815fc27