add "(see below)" to a reference to a new concept

This is because I observed someone reading the tutorial who thought they'd
missed something when they got to the mention of variable bindings.

This patch doesn't reflow the paragraphs so that you can see the semantic
change that I made, and a subsequent patch will reflow this paragraph.

Fixes https://github.com/rust-lang/rust/issues/13570.
reflow with emacs fill-paragraph (fill-column 71)
auto merge of #15166 : zookoatleastauthoritycom/rust/13570-add-see-below-to-a-reference-to-a-new-concept-2, r=huonw

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.