drm/xe/guc: Add kernel-doc for HXG Fast Request

We have kernel-doc for all HXG message types but Fast Request.
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Acked-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240610120411.1768-3-michal.wajdeczko@intel.com

drivers/gpu/drm/xe/abi/guc_messages_abi.h

@@ -91,6 +91,34 @@
 #define GUC_HXG_REQUEST_MSG_0_ACTION		(0xffffu << 0)
 #define GUC_HXG_REQUEST_MSG_n_DATAn		GUC_HXG_MSG_n_PAYLOAD
 
+/**
+ * DOC: HXG Fast Request
+ *
+ * The `HXG Request`_ message should be used to initiate asynchronous activity
+ * for which confirmation or return data is not expected.
+ *
+ * If confirmation is required then `HXG Request`_ shall be used instead.
+ *
+ * The recipient of this message may only use `HXG Failure`_ message if it was
+ * unable to accept this request (like invalid data).
+ *
+ * Format of `HXG Fast Request`_ message is same as `HXG Request`_ except @TYPE.
+ *
+ *  +---+-------+--------------------------------------------------------------+
+ *  |   | Bits  | Description                                                  |
+ *  +===+=======+==============================================================+
+ *  | 0 |    31 | ORIGIN - see `HXG Message`_                                  |
+ *  |   +-------+--------------------------------------------------------------+
+ *  |   | 30:28 | TYPE = `GUC_HXG_TYPE_FAST_REQUEST`_                          |
+ *  |   +-------+--------------------------------------------------------------+
+ *  |   | 27:16 | DATA0 - see `HXG Request`_                                   |
+ *  |   +-------+--------------------------------------------------------------+
+ *  |   |  15:0 | ACTION - see `HXG Request`_                                  |
+ *  +---+-------+--------------------------------------------------------------+
+ *  |...|       | DATAn - see `HXG Request`_                                   |
+ *  +---+-------+--------------------------------------------------------------+
+ */
+
 /**
  * DOC: HXG Event
  *