Change the safe edns-udp-size from 1400 to 1432

Change the safe edns-udp-size from 1400 to 1432

When backporting the Don't Fragment UDP socket option, it was noticed
that the edns-udp-size probing uses 1432 as one of the values to be
probed and the documentation would be recommending 1400 as the safe
value.  As the safe value can be from the 1400-1500 interval, the
documentation has been changed to match the probed value, so we do not
skip it.