closest(): leading combinator selectors not supported

[timmywil]

We intentionally added a guard against selectors needing context. The weirdness around inferring context in a call to .closest led to the decision that this should not be supported. We need a note in the docs explaining that invalid selectors requiring context, such as leading combinators (> *) or positional selectors (:first, :eq, etc.) should not be passed to this function.