mtdblock: Add comment about UBI block devices

There is a surprisingly large number of tutorials
that suggest using mtdblock to mount SquashFS filesystems
on flash devices, including NAND devices.

Given this approach is suboptimal than using UBI, and given
the UBI block device layer was introduced many years ago
specifically with this use case in mind, add a small
comment inviting users and developers to consider UBI block.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210801234509.18774-7-ezequiel@collabora.com