vulkan: Document codegen instructions and code style

Change-Id: I7565506a866b72f4ce31ac6bdd3d97e357497ab0
diff --git a/vulkan/nulldrv/null_driver_gen.cpp b/vulkan/nulldrv/null_driver_gen.cpp
index c5f42b0..10da993 100644
--- a/vulkan/nulldrv/null_driver_gen.cpp
+++ b/vulkan/nulldrv/null_driver_gen.cpp
@@ -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.
 
 #include "null_driver_gen.h"
 #include <algorithm>