radv: add a note about perftest/debug options

Now that all environment variables are documented, it would be
appreciated if we can keep this up-to-date.

[skip ci]

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>

src/amd/vulkan/radv_debug.h

@@ -26,6 +26,7 @@
 
 #include "radv_private.h"
 
+/* Please keep docs/envvars.html up-to-date when you add/remove options. */
 enum {
 	RADV_DEBUG_NO_FAST_CLEARS    =   0x1,
 	RADV_DEBUG_NO_DCC            =   0x2,