vulkan: Document codegen instructions and code style

Change-Id: I7565506a866b72f4ce31ac6bdd3d97e357497ab0
(cherry picked from commit e0d27bb1f3671401c2f6d84a60111f5c695c97d3)
diff --git a/vulkan/nulldrv/null_driver_gen.h b/vulkan/nulldrv/null_driver_gen.h
index ddf4afb..87ff681 100644
--- a/vulkan/nulldrv/null_driver_gen.h
+++ b/vulkan/nulldrv/null_driver_gen.h
@@ -14,9 +14,7 @@
  * limitations under the License.
  */
 
-// This file is generated. Do not edit manually!
-// To regenerate: $ apic template ../api/vulkan.api null_driver.tmpl
-// Requires apic from https://android.googlesource.com/platform/tools/gpu/.
+// WARNING: This file is generated. See ../README.md for instructions.
 
 #ifndef NULLDRV_NULL_DRIVER_H
 #define NULLDRV_NULL_DRIVER_H 1