Documenting misleading DataTransfer API.

tools/dom/docs/docs.json

 {
   "dart.dom.html": {
     "CanvasGradient": {
       "comment": [
         "/**",
         " * An opaque canvas object representing a gradient.",
         " *",
         " * Created by calling [createLinearGradient] or [createRadialGradient] on a",
         " * [CanvasRenderingContext2D] object.",
         " *",
@@ skipping 74 matching lines @@
         " * This context is extended by [CanvasRenderingContext2D] and",
         " * [WebGLRenderingContext].",
         " */"
       ],
       "members": {
         "canvas": [
           "/// Reference to the canvas element to which this context belongs."
         ]
       }
     },
+    "Clipboard": {
+      "members": {
+        "getData": [
+          "/**",
+          "   * Gets the data for the specified MIME type.",
+          "   *",
+          "   * The  data is only available from within a drop operation (such as an",
+          "   * onDrop event) and will return `null` before then.",
+          "   */"
+        ]
+      }
+    },
     "Document": {
       "members": {
         "body": [
           "/// Moved to [HtmlDocument]."
         ],
         "caretRangeFromPoint": [
           "/// Use the [Range] constructor instead."
         ],
         "createElement": [
           "/// Deprecated: use new Element.tag(tagName) instead."
@@ skipping 256 matching lines @@
           "   *",
           "   * is the (more verbose) equivalent of",
           "   *",
           "   *     var request = new HttpRequest.get('http://dartlang.org', (event) => print('Request complete'));",
           "   */"
         ]
       }
     }
   }
 }