Update documentation of how to use roll-dep in DEPS file.

DEPS

 # This file is used to manage the dependencies of the Chromium src repo. It is
 # used by gclient to determine what version of each dependency to check out, and
 # where.
 #
 # For more information, please refer to the official documentation:
 #   https://sites.google.com/a/chromium.org/dev/developers/how-tos/get-the-code
 #
 # When adding a new dependency, please update the top-level .gitignore file
 # to list the dependency's destination directory.
 #
 # -----------------------------------------------------------------------------
 # Rolling deps
 # -----------------------------------------------------------------------------
 # All repositories in this file are git-based, using Chromium git mirrors where
 # necessary (e.g., a git mirror is used when the source project is SVN-based).
 # To update the revision that Chromium pulls for a given dependency:
 #
 #  # Create and switch to a new branch
 #  git new-branch depsroll
-#  # Run roll-dep (provided by depot_tools) giving the dep's path and the
-#  # desired SVN revision number (e.g., third_party/foo/bar and a revision such
-#  # number from Subversion)
-#  roll-dep third_party/foo/bar REVISION_NUMBER
+#  # Run roll-dep (provided by depot_tools) giving the dep's path and optionally
+#  # a regex that will match the line in this file that contains the current
+#  # revision. The script ALWAYS rolls the dependency to the latest revision
+#  # in origin/master. The path for the dep should start with src/.
+#  roll-dep src/third_party/foo_package/src foo_package.git
 #  # You should now have a modified DEPS file; commit and upload as normal
 #  git commit -a
 #  git cl upload
 
 
 vars = {
   # Use this googlecode_url variable only if there is an internal mirror for it.
   # If you do not know, use the full path while defining your new deps entry.
   'googlecode_url': 'http://%s.googlecode.com/svn',
   'webkit_revision': 'ba9d5f19be57c1bd64320e1001425051446d8d87', # from svn revision 200303
@@ skipping 766 matching lines @@
     'pattern': '.',
     'action': [
         'python',
         'src/tools/check_git_config.py',
         '--running-as-hook',
     ],
   },
 ]