mtd-utils: better document --pagesize of mkfs.jffs2

Add a strict explanation '--pagesize' refers to target system's mm page
size.

Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>