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