more perldelta fixups.

more perldelta fixups.

This is my reading from the start of the file through to the start of
=head1 Documentation

* the new 'do' warning isn't mandatory (but merely enabled by default)

* make it clear that the -Di cmd-line switch is new

* remove some optimisations that are trivial or not really optimisations.

* For modules, remove descriptions of bug fixes which are obscure/trivial

* I don't know how the "Compatibility with 5.8 has been restored" text
for threads came about - it was added in the 5.25.1 perldelta, but no
commits around that time seem to support this. So I removed it.

* a few typos etc

* I also added an Internals entry for OP_SPLIT/OP_PUSHRE.