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 type.",
+          "   *",
+          "   * The data is only available from within a drop operation (such as an",
+          "   * [Element.onDrop] event) and will return `null` before the event is",
+          "   * triggered.",
+          "   *",
+          "   * Data transfer is prohibited across domains. If a drag originates",
+          "   * from content from another domain or protocol (HTTP vs HTTPS) then the",
+          "   * data cannot be accessed.",
+          "   *",
+          "   * The [type] can have values such as:",
+          "   *",
+          "   * * `'Text'`",
+          "   * * `'URL'`",
+          "   */"
+        ]
+      }
+    },
     "Document": {
       "members": {
         "body": [
           "/// Moved to [HtmlDocument]."
         ],
         "caretRangeFromPoint": [
           "/// Use the [Range] constructor instead."
         ],
         "createElement": [
           "/// Deprecated: use new Element.tag(tagName) instead."
@@ skipping 472 matching lines @@
           "   *",
           "   * [oldVersion] should match the database's current [version] exactly.",
           "   *",
           "   * * [Database.changeVersion](http://www.w3.org/TR/webdatabase/#dom-database-changeversion) from W3C.",
           "   */"
         ]
       }
     }
   }
 }