Pod::Simple was missing many warnings and errors and thus has been replaced
by Pod::Checker. This is sensible, since the advice given by the
`pod_syntax.t` test when errors occur is that one should use `podchecker` to
investigate errors in detail, thus we should be using Pod::Checker
internally to begin with.
at least the valid ones. podchecker warns about valid
See podchecker docs/*.pod docs/*/*.pod (not the book)
See also GH #1053 to use Pod::Checker in the test and
GH #1054 to relax the fill-column: 100 coda to allow 78
also, to write valid pod.