perldelta: Move some alleged 'enhancements' to 'incompatible changes'

perldelta: Move some alleged 'enhancements' to 'incompatible changes'

I reordered the latter section so the most important things in my view
were earlier in it.

I also extensively reworded the unescaped { section to hopefully better
explain it.

I think the enhancements section should be reordered by importance.  For
example I don't think /xx is the most important enhancement in 5.26.
But I'll let others decide.