Clarify comments about the role of the lex/parse table cache action

Source/bindings/scripts/scripts.gyp

 # Copyright 2014 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.
 
 # Pre-caching steps used internally by the IDL compiler
 #
 # Design doc: http://www.chromium.org/developers/design-documents/idl-build
 
 {
   'includes': [
     'scripts.gypi',
     '../bindings.gypi',
     '../templates/templates.gypi',
   ],
 
   'targets': [
 ################################################################################
   {
-    # A separate pre-caching step is *not required* to use lex/parse table
-    # caching in PLY, as the caches are concurrency-safe.
-    # However, pre-caching ensures that all compiler processes use the cached
-    # files (hence maximizing speed), instead of early processes building the
-    # tables themselves (as they've not yet been written when they start).
+    # This separate pre-caching step is required to use lex/parse table caching
+    # in PLY, since PLY itself does not check if the cache is valid, and may end
+    # up using a stale cache if this step hasn't been run to update it.
+    #
+    # This action's dependencies *is* the cache validation.
     #
     # GN version: //third_party/WebKit/Source/bindings/scripts:cached_lex_yacc_tables
     'target_name': 'cached_lex_yacc_tables',
     'type': 'none',
     'actions': [{
       'action_name': 'cache_lex_yacc_tables',
       'inputs': [
         '<@(idl_lexer_parser_files)',
       ],
       'outputs': [
@@ skipping 32 matching lines @@
         'code_generator_v8.py',
         '<(bindings_scripts_output_dir)',
         '<(bindings_scripts_output_dir)/cached_jinja_templates.stamp',
       ],
       'message': 'Caching bytecode of Jinja templates',
     }],
   },
 ################################################################################
   ],  # targets
 }