[raised by Roland Bless on the mailing list]

Add a note that the same limit applies to IPv4 and IPv6. [If you are in a
homogeneous IPv6 network, you know the larger PMTU of 1280 bytes; if you do not,
using 1280 is dangerous because transition e.g. over v4 tunnels might still
cause fragementation which is not undone at the tunnel exit.]