drm/i915/uapi: Document DRM_I915_QUERY_HWCONFIG_BLOB

UAPI with absolutely no documentation should not have been added -
clarify blob format and content will be described externally.

Fixes: 78e1fb31

 ("drm/i915/uapi: Add query for hwconfig blob")
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Co-developed-by: Jordan Justen <jordan.l.justen@intel.com>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: John Harrison <john.c.harrison@intel.com>
Cc: Jon Ewins <jon.ewins@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220325094916.2186367-1-tvrtko.ursulin@linux.intel.com
[tursulin: Fixed spelling s/meading/meaning/.]