Slightly better description of macro designators.

Closes #3378

doc/rust.md

@@ -510,9 +510,8 @@ For parsing reasons, delimiters must be balanced, but they are otherwise not spe
 
 In the matcher, `$` _name_ `:` _designator_ matches the nonterminal in the
 Rust syntax named by _designator_. Valid designators are `item`, `block`,
-`stmt`, `pat`, `expr`, `ty`, `ident`, `path`, `tt`, `matchers`. The last two
-are the right-hand side and the left-hand side respectively of the `=>` in
-macro rules. In the transcriber, the designator is already known, and so only
+`stmt`, `pat`, `expr`, `ty` (type), `ident`, `path`, `matchers` (lhs of the `=>` in macro rules), 
+`tt` (rhs 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 both the matcher and transcriber, the Kleene star-like operator indicates repetition.