mtd: cmdlinepart: describe mtd ordering

The mtd documentation makes no mention of the useful feature whereby
partitions' logical ordering need not match their physical ordering.
Truncation of parts, skipping of zero sized parts, and handling of
overlapping parts are similarly not mentioned.

This updates the comments at the top of file describing the command
line parsing as currently implemented.   I proposed this in
http://lists.infradead.org/pipermail/linux-mtd/2012-December/045314.html

Signed-off-by: Christopher Cordahi <christophercordahi@nanometrics.ca>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>