Explaining why user groups are sometimes returned by getRestrictions

Title::getRestrictions is assumed to return user rights, but in most
real contexts it actually returns user groups. This is super
confusing and has caused serious permission bugs. Adding an
explanation to the function comments.

Change-Id: I58013ba5e553d1feecacf6bfef066e1569fcc236
Merge "Explaining why user groups are sometimes returned by getRestrictions"