perldelta: move the note about utf8_hop() to the bottom

perldelta: move the note about utf8_hop() to the bottom

A note about a relatively obscure internal API function really doesn't
belong so high up in the "Core Enhancements", before core visible user
changes.

Move this to the bottom, all of the other things in this section are
things normal Perl programmers might actually use/encounter/care
about, whereas this change is only relevant for some XS maintainers.