adding docs to HRElement, testing HTML doc process

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 232 matching lines @@
         " *     document.body.elements.add(elem);",
         " *",
         " * See also:",
         " *",
         " * * [HTML <div> element](http://www.w3.org/TR/html-markup/div.html) from W3C.",
         " * * [Block-level element](http://www.w3.org/TR/CSS2/visuren.html#block-boxes) from W3C.",
         " * * [Inline-level element](http://www.w3.org/TR/CSS2/visuren.html#inline-boxes) from W3C.",
         " */"
       ]
     },
+    "HTMLHRElement": {
+      "comment": [
+        "/**",
+        " * An `<hr>` tag.",
+        " */"
+      ]
+    },
     "HTMLMenuElement": {
       "comment": [
         "/**",
         " * An HTML <menu> element.",
         " *",
         " * A <menu> element represents an unordered list of menu commands.",
         " *",
         " * See also:",
         " *",
         " *  * [Menu Element](https://developer.mozilla.org/en-US/docs/HTML/Element/menu) from MDN.",
@@ skipping 253 matching lines @@
           "   * is the (more verbose) equivalent of",
           "   *",
           "   *     var request = new HttpRequest.get('http://dartlang.org',",
           "   *         (event) => print('Request complete'));",
           "   */"
         ]
       }
     }
   }
 }