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
97519dc52b44a ("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>

Documentation/ABI/testing/sysfs-class-mtd

@@ -229,6 +229,6 @@ KernelVersion:	4.1
 Contact:	linux-mtd@lists.infradead.org
 Description:
 		For a partition, the offset of that partition from the start
-		of the master device in bytes. This attribute is absent on
-		main devices, so it can be used to distinguish between
-		partitions and devices that aren't partitions.
+		of the parent (another partition or a flash device) in bytes.
+		This attribute is absent on flash devices, so it can be used
+		to distinguish them from partitions.