Add a comment about curly braces

Author: nkaretnikov

src/macros/dsl.md

@@ -39,3 +39,6 @@ Output:
 This was a very simple example, but much more complex interfaces have been
 developed, such as [`lazy_static`](https://crates.io/crates/lazy_static) or
 [`clap`](https://crates.io/crates/clap).
+
+Also, note the two pairs of braces in the macro. The outer ones are
+part of the syntax of `macro_rules!`, in addition to `()` or `[]`.