drm/gem: Document that handle_create must be the last step

It published the gem object to userspace, by that point other threads
can guess the id and start using it. And gem IDs are _very_ easy to
guess (it's just an idr).

Since gem objects is the only thing we allow drivers to create
themselves (all the kms/prime/syncobj stuff is handled by the core) no
other functions seem to be in need of this clarification.

Motivated by reviewing the xen-front kms driver.

Cc: Oleksandr Andrushchenko <[e-mail]>
Reviewed-by: Oleksandr Andrushchenko <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_gem.c

···
  * @obj: object to register
  * @handlep: pionter to return the created handle to the caller
  *
- * Create a handle for this object. This adds a handle reference
- * to the object, which includes a regular reference count. Callers
- * will likely want to dereference the object afterwards.
+ * Create a handle for this object. This adds a handle reference to the object,
+ * which includes a regular reference count. Callers will likely want to
+ * dereference the object afterwards.
+ *
+ * Since this publishes @obj to userspace it must be fully set up by this point,
+ * drivers must call this last in their buffer object creation callbacks.
  */
 int drm_gem_handle_create(struct drm_file *file_priv,
 			  struct drm_gem_object *obj,