mtd: nand: Rename the NAND IO iteration helper

Soon a helper for iterating over blocks will be needed (for continuous
read purposes). In order to clarify the intend of this helper, let's
rename it with the "page" wording inside.

While at it, improve the doc and fix a typo.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20240826101412.20644-2-miquel.raynal@bootlin.com