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: 78e1fb3112c0 ("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/.]

include/uapi/drm/i915_drm.h

@@ -2683,6 +2683,9 @@ struct drm_i915_perf_oa_config {
  *
  * The behaviour is determined by the @query_id. Note that exactly what
  * @data_ptr is also depends on the specific @query_id.
+ *
+ * For specific queries see:
+ *  * `GuC HWCONFIG blob uAPI`_
  */
 struct drm_i915_query_item {
 	/** @query_id: The id for this query */
@@ -3135,6 +3138,16 @@ struct drm_i915_query_memory_regions {
 	struct drm_i915_memory_region_info regions[];
 };
 
+/**
+ * DOC: GuC HWCONFIG blob uAPI
+ *
+ * The GuC produces a blob with information about the current device.
+ * i915 reads this blob from GuC and makes it available via this uAPI.
+ *
+ * The format and meaning of the blob content are documented in the
+ * Programmer's Reference Manual.
+ */
+
 /**
  * struct drm_i915_gem_create_ext - Existing gem_create behaviour, with added
  * extension support using struct i915_user_extension.