docs: Update Clover's env variable documentation

Okay, here follows my first commit. 😉 Since !18931 (merged) we have also with clover the possibility to set CLOVER_DEVICE_TYPE. That environment variable is a direct equivalent to rusticl's RUSTICL_DEVICE_TYPE. So this commit adds the presence of that env variable to the Mesa docs.

On the occasion I have added also the CLOVER_DEVICE_VERSION_OVERRIDE and CLOVER_DEVICE_CLC_VERSION_OVERRIDE envs. Additionally there is now mentioned also the experimental IRIS_ENABLE_CLOVER env variable, see !7047 (merged). There might exist some more driver specific environment variables like NOUVEAU_ENABLE_CL but I was not able to find any more substantial information about that so I didn't add it.

Yes, clover is deprecated but it looks that rusticl needs some more time to become ready also for legacy hardware (and drivers like r600).

Finally, the situation around "clover NIR" is somewhat confusing. Currently no support is available on AMD hardware, I was not able to find a corresponding environment variable to enable it. As far as I can tell the support for "clover NIR" on AMD hardware seems to be split around !7357, !7231 and perhaps also !15876. Maybe @illwieckz will find at some point some time to test rusticl also against "clover NIR". 👍

CC @karolherbst

Fixes: !18931 (merged)