style: Document a few more macros.

components/style/gecko/regen_atoms.py

@@ -130,6 +130,7 @@ RULE_TEMPLATE = '''
 '''[1:]
 
 MACRO_TEMPLATE = '''
+/// Returns a static atom by passing the literal string it represents.
 #[macro_export]
 macro_rules! atom {{
 {body}\

components/style/gecko_string_cache/namespace.rs

@@ -11,6 +11,8 @@ use std::borrow::Borrow;
 use std::fmt;
 use std::ops::Deref;
 
+/// In Gecko namespaces are just regular atoms, so this is a simple macro to
+/// forward one macro to the other.
 #[macro_export]
 macro_rules! ns {
     () => {