Documentation: ABI: mtd: describe "offset" more precisely

So far Linux supported only two levels of MTD devices so we didn't need
a very precise description for this sysfs file. With commit
97519dc5

 ("mtd: partitions: add support for subpartitions") there
is support for a tree structure so we should have more precise
description. Using "parent" and "flash device" makes it more accurate.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>