perldelta - Un-TODO 9c7618be

As noted by Karl Williamson, it's very unlikely that any user would have
encountered the bug which this fixed, so it's not worth mentioning in
perldelta.
perldelta - More copy-editing
perldelta for 3428cdc0d3
perldelta - Update entry for b29f65fce6
perldelta - Fill in currently remaining TODO items

9d32676e doesn't need one since it only fixes things that were added post
5.19.3, but [perl #117265] is worth mentioning somewhere so I've added that
to the warnings entry.
perldelta - Remove all but one XXX notices
perldelta - 'nonexistent' should be 'non-existent'