TextFormatterPreview: Add documentation

js/src/forum/components/TextFormatterPreview.js

@@ -2,6 +2,16 @@
 
 import Component from '../../common/Component';
 
+/**
+ * The `TextFormatterPreview` component renders Markdown as HTML using the
+ * TextFormatter library, polling a data source for changes every 50ms. This is
+ * done to prevent expensive redraws on e.g. every single keystroke, while
+ * still retaining the perception of live updates for the user.
+ *
+ * ### Attrs
+ *
+ * - `content` A callback that retrieves the latest content that should be rendered.
+ */
 export default class TextFormatterPreview extends Component {
   view() {
     return <div className="Post-preview" />;