drm/panthor: Document drm_panthor_tiler_heap_destroy::handle validity constraints

Make sure the user is aware that drm_panthor_tiler_heap_destroy::handle
must be a handle previously returned by
DRM_IOCTL_PANTHOR_TILER_HEAP_CREATE.

v4:
- Add Steve's R-b

v3:
- New patch
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240502165158.1458959-6-boris.brezillon@collabora.com

include/uapi/drm/panthor_drm.h

@@ -939,7 +939,11 @@ struct drm_panthor_tiler_heap_create {
  * struct drm_panthor_tiler_heap_destroy - Arguments passed to DRM_IOCTL_PANTHOR_TILER_HEAP_DESTROY
  */
 struct drm_panthor_tiler_heap_destroy {
-	/** @handle: Handle of the tiler heap to destroy */
+	/**
+	 * @handle: Handle of the tiler heap to destroy.
+	 *
+	 * Must be a valid heap handle returned by DRM_IOCTL_PANTHOR_TILER_HEAP_CREATE.
+	 */
 	__u32 handle;
 
 	/** @pad: Padding field, MBZ. */