Added docs to canvaselement. Giving it docs for all members.

tools/dom/templates/html/impl/impl_HTMLCanvasElement.darttemplate

Index: tools/dom/templates/html/impl/impl_HTMLCanvasElement.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_HTMLCanvasElement.darttemplate b/tools/dom/templates/html/impl/impl_HTMLCanvasElement.darttemplate
index 474aca920d362108e784adc354511b91226e5806..9e965e9dd3b6eecfa49bce63e992684e6e8ef38c 100644
--- a/tools/dom/templates/html/impl/impl_HTMLCanvasElement.darttemplate
+++ b/tools/dom/templates/html/impl/impl_HTMLCanvasElement.darttemplate
@@ -14,6 +14,16 @@ $else
   CanvasRenderingContext2D get context2D => getContext('2d');
 $endif
 
+  /**
+   * Returns a new Web GL context for this canvas.
+   *
+   * ## Other resources
+   *
+   * * [WebGL fundamentals]
+   * (http://www.html5rocks.com/en/tutorials/webgl/webgl_fundamentals/) from
+   * HTML5Rocks.
+   * * [WebGL homepage] (http://get.webgl.org/).
+   */
   @SupportedBrowser(SupportedBrowser.CHROME)
   @SupportedBrowser(SupportedBrowser.FIREFOX)
   @Experimental()