drm/xe/configfs: Fix documentation warning

drm/xe/configfs: Fix documentation warning

Fix this warning while building the documentation:

	Documentation/gpu/xe/xe_configfs:9: drivers/gpu/drm/xe/xe_configfs.c:138:
	WARNING: Definition list ends without a blank line; unexpected unindent.

That also makes it better formatted in the output.

While at it, also fix the underline length in "Overview".

Fixes: e2b33fce5eb0 ("drm/xe/configfs: Improve documentation steps")
Reviewed-by: Raag Jadav <raag.jadav@intel.com>
Link: https://lore.kernel.org/r/20250911-wa-bb-cmds-v4-2-c8f7e48f7eae@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>