perldiag: Rewrap an entry for better splain output

perldiag: Rewrap an entry for better splain output

The splain utility (aka diagnostics.pm) blindly strips out pod formatting
without rewrapping the text (I think rewrapping would be overkill).  This
means that formatting like ‘L<Note [5] in
perlrecharclass|perlrecharclass/[5]>’ turns into a simple ‘Note [5] in
perlrecharclass’, which can mess up the indentation.

Rewrap the entry to compensate, plus a few other tweaks to improve
the appearance of the right margin.