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 rust-lang#13570.

src/doc/tutorial.md

@@ -474,7 +474,7 @@ often convenient to use a block expression for each case, in which case
 the commas are optional as shown below. Literals are valid patterns and
 match only their own value. A single arm may match multiple different
 patterns by combining them with the pipe operator (`|`), so long as every
-pattern binds the same set of variables. Ranges of numeric literal
+pattern binds the same set of variables (see "destructuring" below). Ranges of numeric literal
 patterns can be expressed with two dots, as in `M..N`. The underscore
 (`_`) is a wildcard pattern that matches any single value. (`..`) is a
 different wildcard that can match one or more fields in an `enum` variant.