perms_syncer: fix logged counts and move up docstring

Created by: unknwon

The call of SetRepoPendingPermissions would reset p.UserIDs in place, so we were only logging the count of pending users.

Also move up docstring of GrantPendingPermissions to its interface, I think it was overlooked.

Discovered while working on https://github.com/sourcegraph/customer/issues/993.

Test plan

Manual testing and unit tests.