add `meta` designator to macro reference

src/doc/reference.md

@@ -538,8 +538,9 @@ balanced, but they are otherwise not special.
 In the matcher, `$` _name_ `:` _designator_ matches the nonterminal in the Rust
 syntax named by _designator_. Valid designators are `item`, `block`, `stmt`,
 `pat`, `expr`, `ty` (type), `ident`, `path`, `tt` (either side of the `=>`
-in macro rules). In the transcriber, the designator is already known, and so
-only the name of a matched nonterminal comes after the dollar sign.
+in macro rules), and `meta` (contents of an attribute). In the transcriber, the
+designator is already known, and so only the name of a matched nonterminal comes
+after the dollar sign.
 
 In both the matcher and transcriber, the Kleene star-like operator indicates
 repetition. The Kleene star operator consists of `$` and parentheses, optionally