docs: document AMD_DEBUG variable

See https://gitlab.freedesktop.org/mesa/mesa/issues/2022

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3492>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3492>
1 file changed