Document just-in-time methods for handles

Summary: Fixes T7689. I'm not going to go clean up all the rest of the `loadViewerHandles()` calls right now since a lot of them are kind of a pain and they aren't really hurting anything so it doesn't feel very leveraged, but at least deprecate it and document the new hotness.

Test Plan:
have a look
in a book
reading rainbow

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7689

Differential Revision: https://secure.phabricator.com/D12343

src/applications/base/controller/PhabricatorController.php

@@ -427,6 +427,11 @@ abstract class PhabricatorController extends AphrontController {
     return $response;
   }
 
+  /**
+   * WARNING: Do not call this in new code.
+   *
+   * @deprecated See "Handles Technical Documentation".
+   */
   protected function loadViewerHandles(array $phids) {
     return id(new PhabricatorHandleQuery())
       ->setViewer($this->getRequest()->getUser())