Document zip(with:) on optionals

Sources/JSONAPIOpenAPI/Optional+ZipWith.swift

@@ -5,6 +5,9 @@
 //  Created by Mathew Polzin on 1/19/19.
 //
 
+/// Zip two optionals together with the given operation performed on
+/// the unwrapped contents. If either optional is nil, the zip
+/// yields nil.
 func zip<X, Y, Z>(_ left: X?, _ right: Y?, with fn: (X, Y) -> Z) -> Z? {
 	return left.flatMap { lft in right.map { rght in fn(lft, rght) }}
 }