Expand documentation for `identity`

Originally [from ReactiveCocoa](https://github.com/ReactiveCocoa/ReactiveCocoa/blob/ff67dff254204bdf071299ecdf21f41581a18d22/ReactiveCocoa/Identity.swift).

/cc https://github.com/ReactiveCocoa/ReactiveCocoa/pull/1406#discussion_r14872010