13570 add see below to a reference to a new concept 2

This is the same patch as submitted to https://github.com/rust-lang/rust/issues/13570 and https://github.com/rust-lang/rust/pull/14124, but with @pnkfelix's comment (https://github.com/rust-lang/rust/pull/14124#issuecomment-42797536) addressed, and with reflow as a separate commit. I'm submitting it in case @steveklabnik hasn't yet merged a rewrite of the tutorial (https://github.com/rust-lang/rust/issues/13570#issuecomment-46864789), in which case this patch might as well be merged into the old tutorial.