mtd: adapt misleading comment in mtd_oob_ops structure

A comment in the kernel doc of the mtd_oob_ops structure tells that it
is not possible to write more than one page with OOB. This is actually
true for only a few MTD devices like 'onenand' but it is definitely not
a general limitation. While this would benefit to be handled elsewhere
either by the MTD layer or by the limited drivers, let's update this
comment to reflect the reality.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>

include/linux/mtd/mtd.h

@@ -67,9 +67,11 @@ struct mtd_erase_region_info {
  * @datbuf:	data buffer - if NULL only oob data are read/written
  * @oobbuf:	oob data buffer
  *
- * Note, it is allowed to read more than one OOB area at one go, but not write.
- * The interface assumes that the OOB write requests program only one page's
- * OOB area.
+ * Note, some MTD drivers do not allow you to write more than one OOB area at
+ * one go. If you try to do that on such an MTD device, -EINVAL will be
+ * returned. If you want to make your implementation portable on all kind of MTD
+ * devices you should split the write request into several sub-requests when the
+ * request crosses a page boundary.
  */
 struct mtd_oob_ops {
 	unsigned int	mode;