Add documentation for RADV_UMR_EXTRA_ARGS
What does this MR do and why?
These commits add some simple documentation for the RADV_UMR_EXTRA_ARGS variable in in RADV's debugging hangs page
and adds the variable in the envvar page right under the RADV_DEBUG envvar
I'm doing this because it is valuable for systems with multiple gpus and systems like mine where my gpu isn't card0
and instead card1 so RADV_DEBUG=hang doesn't work properly without setting RADV_UMR_EXTRA_ARGS="--instance 1"
Edited by Jacob Mungle