media: vb2: Fix buf_out_validate documentation

The .buf_out_validate callback is mandatory for OUTPUT
queues. Mark it as such in the callback's doc.

Fixes: 28d77c21 ("media: vb2: add buf_out_validate callback")
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

include/media/videobuf2-core.h

@@ -345,7 +345,8 @@ struct vb2_buffer {
  *			waiting for a new buffer to arrive.
  * @buf_out_validate:	called when the output buffer is prepared or queued
  *			to a request; drivers can use this to validate
- *			userspace-provided information; optional.
+ *			userspace-provided information; this is required only
+ *			for OUTPUT queues.
  * @buf_init:		called once after allocating a buffer (in MMAP case)
  *			or after acquiring a new USERPTR buffer; drivers may
  *			perform additional buffer-related initialization;