Bluetooth: Mesh: Clearly document the magic 18 byte minimum length

This makes it clear why 18 is a valid minimum network PDU length to
enforce. This is particularly important since as of writing this patch
there's at least one PTS test case which sends too small PDUs, which
could potentially lead to people thinking the implementation is at
fault (it's not).

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>