drm/xe/configfs: Add internal API to documentation

Add the internal configfs API like is done with other parts of the
driver. Also ensure the correct number of '=' chars are used for the
header.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://lore.kernel.org/r/20250528-engine-mask-v4-5-f4636d2a890a@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>

Author: lucasdemarchi

Documentation/gpu/xe/xe_configfs.rst

@@ -2,9 +2,15 @@
 
 .. _xe_configfs:
 
-============
+===========
 Xe Configfs
-============
+===========
 
 .. kernel-doc:: drivers/gpu/drm/xe/xe_configfs.c
    :doc: Xe Configfs
+
+Internal API
+============
+
+.. kernel-doc:: drivers/gpu/drm/xe/xe_configfs.c
+   :internal: