Add doc for FormBuilder#rich_text_area

Using the style of other ActionView::Helpers::FormBuilder methods that
delegate to another method.

actiontext/app/helpers/action_text/tag_helper.rb

@@ -83,6 +83,13 @@ def rich_text_area(object_name, method, options = {})
   end
 
   class FormBuilder
+    # Wraps ActionView::Helpers::FormHelper#rich_text_area for form builders:
+    #
+    #   <%= form_with model: @message do |f| %>
+    #     <%= f.rich_text_area :content %>
+    #   <% end %>
+    #
+    # Please refer to the documentation of the base helper for details.
     def rich_text_area(method, options = {})
       @template.rich_text_area(@object_name, method, objectify_options(options))
     end