perldelta - Fill in the remaining stuff that needed to be done still

Commit 8fe3c67 was actually part of merge e82485c, covered already by
31d5dc6.
perldelta - Standardize references to other versions of Perl
perldelta - Remove all but one XXX notices

Also, move a performance bug fix to the Performance Enhancements section
and mention new test scripts in the Testing section.