GPU frequencies now reset after job is finished

Add step_unconfigure_hardware() to GRES plugin API
Update test39.18 regarding links.
Update GRES docs.
Update docs related to links.
Document GPU frequency resetting behavior.
Specify what the default is for GpuFreqDef.
Move NVML init and shutdown to configure() and unconfigure().
Get rid of superfluous `!= 0`-style statements.
Print note when GPU index != minor number.
Clean up various formatting and other errors.

bug 5520