Update gclient sync docs to reflect reality + add an example for the most common usage.

Review URL: http://codereview.chromium.org/456008

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@33390 0039d316-1c4b-4281-b951-d872f2087c98

gclient.py

@@ -128,7 +128,6 @@ usage: gclient %(cmd)s [options] [--] [svn update options/args]
 Valid options:
   --force                 : force update even for unchanged modules
   --nohooks               : don't run the hooks after the update is complete
-  --revision REV      : update/checkout all solutions with specified revision
   --revision SOLUTION@REV : update given solution to specified revision
   --deps PLATFORM(S)      : sync deps for the given platform(s), or 'all'
   --verbose               : output additional diagnostics
@@ -141,6 +140,8 @@ Examples:
   gclient %(cmd)s --force
       update files from SVN according to current configuration, for
       all modules (useful for recovering files deleted from local copy)
+  gclient %(cmd)s --revision src@31000
+      update src directory to r31000
 """)
 
 COMMAND_USAGE_TEXT = {