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

lib/sass/tree/debug_node.rb

@@ -1,6 +1,12 @@
 module Sass
   module Tree
+    # A dynamic node representing a Sass `@debug` statement.
+    #
+    # @see Sass::Tree
     class DebugNode < Node
+      # @param expr [Script::Node] The expression to print
+      # @param options [Hash<Symbol, Object>] An options hash;
+      #   see [the Sass options documentation](../../Sass.html#sass_options)
       def initialize(expr, options)
         @expr = expr
         super(options)
@@ -8,6 +14,10 @@ module Sass
 
       protected
 
+      # Prints the expression to STDERR.
+      #
+      # @param environment [Sass::Environment] The lexical environment containing
+      #   variable and mixin values
       def _perform(environment)
         res = @expr.perform(environment)
         if filename