If --force is specified when updating, remove unversioned directories

gclient.py

Index: gclient.py
===================================================================
--- gclient.py	(revision 121254)
+++ gclient.py	(working copy)
@@ -1280,7 +1280,12 @@
 def CMDsync(parser, args):
   """Checkout/update all modules."""
   parser.add_option('-f', '--force', action='store_true',
-                    help='force update even for unchanged modules')
+                    help='Forces update even for unchanged modules. Also '
+                         'removes from the working copy directories which have '
+                         'been removed from the repository in this update and '
+                         'hence are untracked. Directories are removed even if '
+                         'explicitly ignored, to allow dependencies to be '
+                         'installed in their place.')
   parser.add_option('-n', '--nohooks', action='store_true',
                     help='don\'t run hooks after the update is complete')
   parser.add_option('-r', '--revision', action='append',
@@ -1300,10 +1305,11 @@
                     help='skips any safesync_urls specified in '
                          'configured solutions and sync to head instead')
   parser.add_option('-D', '--delete_unversioned_trees', action='store_true',
-                    help='delete any dependency that have been removed from '
-                         'last sync as long as there is no local modification. '
-                         'Coupled with --force, it will remove them even with '
-                         'local modifications')
+                    help='Deletes from the working copy any dependencies that '
+                         'have been removed since the last sync, as long as '
+                         'there are no local modifications. When used with '
+                         '--force, such dependencies are removed even if they '
+                         'have local modifications.')
   parser.add_option('-R', '--reset', action='store_true',
                     help='resets any local changes before updating (git only)')
   parser.add_option('-M', '--merge', action='store_true',