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

Source/bindings/scripts/scripts.gyp

Index: Source/bindings/scripts/scripts.gyp
diff --git a/Source/bindings/scripts/scripts.gyp b/Source/bindings/scripts/scripts.gyp
index d8d8687171305908ed0762ec6931d35c70778e7b..bb5319022096115915b24fde8087490acf6cecd4 100644
--- a/Source/bindings/scripts/scripts.gyp
+++ b/Source/bindings/scripts/scripts.gyp
@@ -16,11 +16,11 @@
   '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',