drm/doc: document drm_event and its types

Convert struct drm_event to a kernel doc comment. Link to the
generic DRM event types. Add a basic description of each event
type.

Signed-off-by: Simon Ser <contact@emersion.fr>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20230717093032.600773-1-contact@emersion.fr

include/uapi/drm/drm.h

@@ -1202,25 +1202,50 @@ extern "C" {
 #define DRM_COMMAND_BASE                0x40
 #define DRM_COMMAND_END			0xA0
 
-/*
- * Header for events written back to userspace on the drm fd.  The
- * type defines the type of event, the length specifies the total
- * length of the event (including the header), and user_data is
- * typically a 64 bit value passed with the ioctl that triggered the
- * event.  A read on the drm fd will always only return complete
- * events, that is, if for example the read buffer is 100 bytes, and
- * there are two 64 byte events pending, only one will be returned.
+/**
+ * struct drm_event - Header for DRM events
+ * @type: event type.
+ * @length: total number of payload bytes (including header).
  *
- * Event types 0 - 0x7fffffff are generic drm events, 0x80000000 and
- * up are chipset specific.
+ * This struct is a header for events written back to user-space on the DRM FD.
+ * A read on the DRM FD will always only return complete events: e.g. if the
+ * read buffer is 100 bytes large and there are two 64 byte events pending,
+ * only one will be returned.
+ *
+ * Event types 0 - 0x7fffffff are generic DRM events, 0x80000000 and
+ * up are chipset specific. Generic DRM events include &DRM_EVENT_VBLANK,
+ * &DRM_EVENT_FLIP_COMPLETE and &DRM_EVENT_CRTC_SEQUENCE.
  */
 struct drm_event {
 	__u32 type;
 	__u32 length;
 };
 
+/**
+ * DRM_EVENT_VBLANK - vertical blanking event
+ *
+ * This event is sent in response to &DRM_IOCTL_WAIT_VBLANK with the
+ * &_DRM_VBLANK_EVENT flag set.
+ *
+ * The event payload is a struct drm_event_vblank.
+ */
 #define DRM_EVENT_VBLANK 0x01
+/**
+ * DRM_EVENT_FLIP_COMPLETE - page-flip completion event
+ *
+ * This event is sent in response to an atomic commit or legacy page-flip with
+ * the &DRM_MODE_PAGE_FLIP_EVENT flag set.
+ *
+ * The event payload is a struct drm_event_vblank.
+ */
 #define DRM_EVENT_FLIP_COMPLETE 0x02
+/**
+ * DRM_EVENT_CRTC_SEQUENCE - CRTC sequence event
+ *
+ * This event is sent in response to &DRM_IOCTL_CRTC_QUEUE_SEQUENCE.
+ *
+ * The event payload is a struct drm_event_crtc_sequence.
+ */
 #define DRM_EVENT_CRTC_SEQUENCE	0x03
 
 struct drm_event_vblank {