drm/xe/configfs: Improve doc for ctx_restore* attributes

drm/xe/configfs: Improve doc for ctx_restore* attributes

Spell out the syntax instead of only using examples. Particularly
important the <engine-class> part since that's different than
engines_allowed and may confuse users. The same batch buffer is used for
all engines of a certain class.

Cc: Raag Jadav <raag.jadav@intel.com>
Reviewed-by: Raag Jadav <raag.jadav@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Fixes: e2a9854d806e ("drm/xe/configfs: Allow to select by class only")
Link: https://lore.kernel.org/r/20250924152709.659483-4-lucas.demarchi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>