drm/i915/uapi: convert i915_user_extension to kernel doc

Add some example usage for the extension chaining also, which is quite
nifty.

v2: (Daniel)
  - clarify that the name is just some integer, also document that the
    name space is not global
v3: prefer kernel-doc references for structs
Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Dave Airlie <airlied@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Cc: mesa-dev@lists.freedesktop.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20210419105741.27844-3-matthew.auld@intel.com

include/uapi/drm/i915_drm.h

@@ -62,8 +62,8 @@ extern "C" {
 #define I915_ERROR_UEVENT		"ERROR"
 #define I915_RESET_UEVENT		"RESET"
 
-/*
- * i915_user_extension: Base class for defining a chain of extensions
+/**
+ * struct i915_user_extension - Base class for defining a chain of extensions
  *
  * Many interfaces need to grow over time. In most cases we can simply
  * extend the struct and have userspace pass in more data. Another option,
@@ -76,12 +76,58 @@ extern "C" {
  * increasing complexity, and for large parts of that interface to be
  * entirely optional. The downside is more pointer chasing; chasing across
  * the __user boundary with pointers encapsulated inside u64.
+ *
+ * Example chaining:
+ *
+ * .. code-block:: C
+ *
+ *	struct i915_user_extension ext3 {
+ *		.next_extension = 0, // end
+ *		.name = ...,
+ *	};
+ *	struct i915_user_extension ext2 {
+ *		.next_extension = (uintptr_t)&ext3,
+ *		.name = ...,
+ *	};
+ *	struct i915_user_extension ext1 {
+ *		.next_extension = (uintptr_t)&ext2,
+ *		.name = ...,
+ *	};
+ *
+ * Typically the struct i915_user_extension would be embedded in some uAPI
+ * struct, and in this case we would feed it the head of the chain(i.e ext1),
+ * which would then apply all of the above extensions.
+ *
  */
 struct i915_user_extension {
+	/**
+	 * @next_extension:
+	 *
+	 * Pointer to the next struct i915_user_extension, or zero if the end.
+	 */
 	__u64 next_extension;
+	/**
+	 * @name: Name of the extension.
+	 *
+	 * Note that the name here is just some integer.
+	 *
+	 * Also note that the name space for this is not global for the whole
+	 * driver, but rather its scope/meaning is limited to the specific piece
+	 * of uAPI which has embedded the struct i915_user_extension.
+	 */
 	__u32 name;
-	__u32 flags; /* All undefined bits must be zero. */
-	__u32 rsvd[4]; /* Reserved for future use; must be zero. */
+	/**
+	 * @flags: MBZ
+	 *
+	 * All undefined bits must be zero.
+	 */
+	__u32 flags;
+	/**
+	 * @rsvd: MBZ
+	 *
+	 * Reserved for future use; must be zero.
+	 */
+	__u32 rsvd[4];
 };
 
 /*