docs(panel): fix casing and add clarity to documentation on mdPanelRef (#10502)

Author: bryanrideshark

src/components/panel/panel.js

@@ -111,6 +111,7 @@ angular
  *   }
  *
  *   function PanelMenuCtrl(mdPanelRef) {
+ *     // The controller is provided with an import named 'mdPanelRef'
  *     this.closeMenu = function() {
  *       mdPanelRef && mdPanelRef.close();
  *     };
@@ -235,7 +236,8 @@ angular
  *   - `locals` - `{Object=}`: An object containing key/value pairs. The keys
  *     will be used as names of values to inject into the controller. For
  *     example, `locals: {three: 3}` would inject `three` into the controller,
- *     with the value 3.
+ *     with the value 3. 'mdPanelRef' is a reserved key, and will always 
+ *     be set to the created MdPanelRef instance.
  *   - `resolve` - `{Object=}`: Similar to locals, except it takes promises as
  *     values. The panel will not open until all of the promises resolve.
  *   - `attachTo` - `{(string|!angular.JQLite|!Element)=}`: The element to