15.13.1: Clarify how potentially-applicable methods are derived for method refs

15.13.1 delegates to 15.12.2.1 for identifying the potentially-applicable methods of a method reference expression. The correspondence isn't as clear as it could be:

- "appropriate arity" suggests that the results will have the given arity (n), not variable-arity methods of smaller arity

- "appropriate type argument arity" suggests that the results will have the number of type arguments given by "TypeArguments", not non-generic methods.

A better way to express the relationship is describe it "as if for a method invocation with name ___, arity ___, and type arguments ___".

These comments apply to multiple bullets in the potentially-applicable method section of 15.13.1.