Improve doc for gclient.pach_projects mapping.

recipe_modules/gclient/config.py

 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import types
 
 from recipe_engine.config import config_item_context, ConfigGroup, BadConf
 from recipe_engine.config import ConfigList, Dict, Single, Static, Set, List
 
 from . import api as gclient_api
@@ skipping 49 matching lines @@
     # If value is None, the property value will be applied to
     # solutions[0].revision. Otherwise, it will be applied to
     # solutions[0].custom_vars['custom_var_name']
     parent_got_revision_mapping = Dict(hidden=True),
     delete_unversioned_trees = Single(bool, empty_val=True, required=False),
 
     # Maps patch_project to (solution/path, revision).
     #  - solution/path is then used to apply patches as patch root in
     #    bot_update.
     #  - if revision is given, it's passed verbatim to bot_update for
-    #    corresponding dependency.
+    #    corresponding dependency. Otherwise (ie None), the patch will be
+    #    applied on top of version pinned in DEPS.
     # This is essentially a whitelist of which projects inside a solution
     # can be patched automatically by bot_update based on PATCH_PROJECT
     # property.
     # For example, bare chromium solution has this entry in patch_projects
     #     'angle/angle': ('src/third_party/angle', 'HEAD')
     # then a patch to Angle project can be applied to a chromium src's
     # checkout after first updating Angle's repo to its master's HEAD.
     patch_projects = Dict(value_type=tuple, hidden=True),
 
     # Check out refs/branch-heads.
@@ skipping 578 matching lines @@
   soln = c.solutions.add()
   soln.name = 'gerrit-test-cq-normal'
   soln.url = 'https://chromium.googlesource.com/playground/gerrit-cq/normal.git'
 
 # TODO(phajdan.jr): Move to proper repo and add coverage.
 @config_ctx()
 def valgrind(c):  # pragma: no cover
   """Add Valgrind binaries to the gclient solution."""
   c.solutions[0].custom_deps['src/third_party/valgrind'] = \
     ChromiumGitURL(c, 'chromium', 'deps', 'valgrind', 'binaries')