Correct IPv4 MTU threshhold for clearing DF bit

The DF bit should only be cleared for generated IPv4 packets that are
smaller than or equal to 1260 bytes, as this would correspond to an IPv6
packet smaller than or equal to 1280 bytes. Therefore, PMTUD can operate
normally across the translator with IPv4 paths >1260 (IPv6 paths >1280),
so there's no need to clear the DF bit.
Remove text regarding v6 frags to v4 frags

This text is out of place, as it has nothing to do with accomodating
differences in minimum MTU betwene IPv4 and IPv6. Rather, it simply
described how the stateless translator should translate preexisting
fragments. This is however redundant, as the process is described in
RFC 6145 section 5.1.1.

Besides, the text was a bit confusing. It starts talking about "an IPv6
packet" singular, but finishes talking about "all of the IPv6
fragments" plural.
More description of how the new approach works
Add a reference to the relevant section.
Explain how ICMPv6 PTB is being clamped up to 1280
Correct IPv4 packet size resulting in DF=0

Also remove special handling for small IPv4 packets. I do not see the
point in adding complexity to the text for these packets, as the DF flag
should be irrelevant when they are smaller or equal to the minimum IPv4
link MTU.