[Sass] Convert Sass::Tree::MixinDefNode docs to YARD.

lib/sass/tree/mixin_def_node.rb

@@ -1,14 +1,25 @@
 module Sass
   module Tree
+    # A dynamic node representing a mixin definition.
+    #
+    # @see Sass::Tree
     class MixinDefNode < Node
+      # @param name [String] The mixin name
+      # @param args [Array<(String, Script::Node)>] The arguments for the mixin.
+      #   Each element is a tuple containing the name of the argument
+      #   and the parse tree for the default value of the argument
       def initialize(name, args, options)
         @name = name
         @args = args
         super(options)
       end
 
-      private
+      protected
 
+      # Loads the mixin into the environment.
+      #
+      # @param environment [Sass::Environment] The lexical environment containing
+      #   variable and mixin values
       def _perform(environment)
         environment.set_mixin(@name, Sass::Mixin.new(@name, @args, environment, children))
         []