[tools][gen] Describe how to regenerate files in the header

Change-Id: I9b0b9e4ecad25122b4b046c1283333aee48ee246
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/147801
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Kokoro: Ben Clayton <bclayton@google.com>
diff --git a/src/tint/lang/core/extension.h b/src/tint/lang/core/extension.h
index 0aac98a..da1c9f4 100644
--- a/src/tint/lang/core/extension.h
+++ b/src/tint/lang/core/extension.h
@@ -13,11 +13,12 @@
 // limitations under the License.
 
 ////////////////////////////////////////////////////////////////////////////////
-// File generated by tools/src/cmd/gen
-// using the template:
+// File generated by 'tools/src/cmd/gen' using the template:
 //   src/tint/lang/core/extension.h.tmpl
 //
-// Do not modify this file directly
+// To regenerate run: './tools/run gen'
+//
+//                       Do not modify this file directly
 ////////////////////////////////////////////////////////////////////////////////
 
 #ifndef SRC_TINT_LANG_CORE_EXTENSION_H_