Improve description of "Show Filetree"

Summary: Clearer?

Test Plan: Read it.

Reviewers: nh

Reviewed By: nh

CC: aran

Differential Revision: https://secure.phabricator.com/D4908

src/applications/settings/panel/PhabricatorSettingsPanelDiffPreferences.php

@@ -44,8 +44,10 @@ final class PhabricatorSettingsPanelDiffPreferences
               1 => pht('Enable Filetree'),
             ))
           ->setCaption(
-            pht("When looking at a revision or commit, show affected files ".
+            pht("When looking at a revision or commit, enable a sidebar ".
-                "in a sidebar.")))
+                "showing affected files. You can press %s to show or hide ".
+                "the sidebar.",
+                phutil_tag('tt', array(), 'f'))))
       ->appendChild(
         id(new AphrontFormSubmitControl())
           ->setValue(pht('Save Preferences')));