NVML linking fails
The fix for #2087 cleared GMX_EXTRA_LIBRARIES after NVML handling set it, so linking with NVML fails. Found by Magnus Lundborg on local tcbs machines and diagnosed by Mark.
Either we should not clear such variables, clear them earlier, or use some other mechanism for linking to NVML.
Initialize GMX_*_LIBRARIES cmake variables earlier
NVML linking got broken by the way the introduction of separate
variables also set them to empty initial values, but did so after
gmxManageGPU.cmake tried to set GMX_EXTRA_LIBRARIES.
Made minor improvements to the documentation