reference: fix definition of :tt

The reference says that $x:tt matches "either side of the `=>` in macro_rules` which is technically true but completely uninformative. This changes that bullet point to what the book says (a single token or sequence of token trees inside brackets).

src/doc/reference.md

@@ -603,7 +603,8 @@ syntax named by _designator_. Valid designators are:
 * `ty`: a [type](#types)
 * `ident`: an [identifier](#identifiers)
 * `path`: a [path](#paths)
-* `tt`: either side of the `=>` in macro rules
+* `tt`: a token tree (a single [token](#tokens) or a sequence of token trees surrounded
+  by matching `()`, `[]`, or `{}`)
 * `meta`: the contents of an [attribute](#attributes)
 
 In the transcriber, the