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

Source/bindings/scripts/BUILD.gn

Index: Source/bindings/scripts/BUILD.gn
diff --git a/Source/bindings/scripts/BUILD.gn b/Source/bindings/scripts/BUILD.gn
index cdc50836c4c8201164e4ea38b7bf496205f46400..2df902886a312f021d0319e929aa28a913a8a1e4 100644
--- a/Source/bindings/scripts/BUILD.gn
+++ b/Source/bindings/scripts/BUILD.gn
@@ -8,11 +8,12 @@ import("//third_party/WebKit/Source/bindings/templates/templates.gni")
 
 visibility = "//third_party/WebKit/*"
 
-# 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 thus 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.
 #
 # GYP version: scripts.gyp:cached_lex_yacc_tables
 action("cached_lex_yacc_tables") {