DO NOT MERGE Correct the PackageManager.checkSignatures documentation.

Make the documentation match the current behavior since change ID
Ibdeff170bd386d723f774136b18e0ad59d9cdabb.

Will add @deprecated tags back in in a separate CL. This is just to get
things in the same state as master, following the build breakage mishap.

Change-Id: I47c53700892473061093002c32ad8f095b5813d1

Do not merge to master; an equivalent CL has already been committed there.