Tweak prose to account for ident-token

Author: kbabbitt

css-mixins-1/Overview.bs

@@ -950,7 +950,8 @@ and consists of a name,
 a list of [=mixin parameters=],
 and a [=mixin body=].
 (Identical to ''@function'',
-save that it lacks a [=custom function/return type=].)
+save that it lacks a [=custom function/return type=],
+and that the parameter list and enclosing parentheses are optional.)
 
 <pre class="prod def">
 	<<@mixin>> = @mixin [<<function-token>> <<function-parameter>>#? ) | <<ident-token>> ]
@@ -974,16 +975,19 @@ it is invalid within the body of a [=style rule=].
 <h4 id=mixin-prelude>
 The Mixin Prelude</h4>
 
-The <<function-token>> production
+The <<function-token>> or <<ident-token>> production
 must start with two dashes (U+002D HYPHEN-MINUS),
 similar to <<dashed-ident>>,
 or else the definition is invalid.
 
-The name of the resulting [=mixin=] is given by the name of the <<function-token>>.
+The name of the resulting [=mixin=] is given by the name of the <<function-token>> or <<ident-token>>.
 The optional [=mixin parameters=]
 are given by the <<function-parameter>> values
 (defaulting to an empty set).
 
+If an <<ident-token>> is used,
+it is equivalent to a <<function-token>> followed by an empty parameter list.
+
 The name of a ''@mixin'' rule is a [=tree-scoped name=].
 If more than one ''@mixin'' exists for a given name,
 then the rule in the stronger cascade layer wins,