Add documentation for HttpRequest.

sdk/lib/html/templates/html/impl/impl_XMLHttpRequest.darttemplate

Index: sdk/lib/html/templates/html/impl/impl_XMLHttpRequest.darttemplate
===================================================================
--- sdk/lib/html/templates/html/impl/impl_XMLHttpRequest.darttemplate	(revision 15801)
+++ sdk/lib/html/templates/html/impl/impl_XMLHttpRequest.darttemplate	(working copy)
@@ -4,11 +4,34 @@
 
 part of html;
 
-/// @domName $DOMNAME
+/// HttpRequest is a utility for retrieving data from a URL.
+///
+/// HttpRequest can be used to obtain data from http, ftp, as well as file
+/// protocols. 
+///
+/// For example, suppose we're developing this Dart documentation page, and we
+/// wish to retrieve the html of the top level page and print it out.
+/// The easiest way to do that would be:
+///
+///     var httpRequest = HttpRequest.get('http://api.dartlang.org', (request) => print(request.responseText));
+///
+/// It is important to note that with the default behavior of this class, your
+/// code making the request should be served from the same origin (domain name,
+/// port, and application layer protocol) as the url you are trying to access
+/// with HttpRequest. However, there are ways to 
+/// [get around this restriction](http://www.dartlang.org/articles/json-web-service/#note-on-jsonp).
+///
+/// See also: 
+/// [Dart article on using HttpRequests](http://www.dartlang.org/articles/json-web-service/#getting-data),
+/// [JS XMLHttpRequest](https://developer.mozilla.org/en-US/docs/DOM/XMLHttpRequest),
+/// and [Using XMLHttpRequest](https://developer.mozilla.org/en-US/docs/DOM/XMLHttpRequest/Using_XMLHttpRequest)
+/// @domName $DOMNAME; @docsEditable true
 class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
+  /// @docsEditable true  
   factory $CLASSNAME.get(String url, onComplete($CLASSNAME request)) =>
       _$(CLASSNAME)FactoryProvider.create$(CLASSNAME)_get(url, onComplete);
 
+  /// @docsEditable true  
   factory $CLASSNAME.getWithCredentials(String url,
       onComplete($CLASSNAME request)) =>
       _$(CLASSNAME)FactoryProvider.create$(CLASSNAME)_getWithCredentials(url,