Expand documentation for `identity`
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
mpurland/Swiftz,tranhieutt/Swiftz,gaurav1981/Swiftz,ikesyo/Swiftz,hffmnn/Swiftz,typelift/swiftz,mathcamp/swiftz,ryanbooker/Swiftz,Khan/swiftz,typelift/swiftz,CodaFi/swiftz,CodaFi/swiftz,CodaFi/swiftz,mathcamp/swiftz,ryanbooker/Swiftz,mpurland/Swiftz,hffmnn/Swiftz,mpurland/Swiftz,gaurav1981/Swiftz,Khan/swiftz,gaurav1981/Swiftz,ikesyo/Swiftz,tranhieutt/Swiftz,ikesyo/Swiftz,typelift/swiftz