[lib] using Pod::Checker to check pod

Pod::Simple was missing many warnings and errors and thus has been replaced
by Pod::Checker.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.
[tools] fixing errors in pod found by podchecker
[t] correcting pod errors found by podchecker
[string/encoding] Adding missing =back pod statements

This fixes pod errors found by podchecker
[pmc] correcting pod errors found by podchecker
[runcore] correcting pod errors found by podchecker
[platform] correcting pod errors found by podchecker
[ops] adding missing =back to pod; found by podchecker
[io] correcting pod error found by podchecker
[interp] adding missing =back in pod found by podchecker
[gc] correcting pod errors found by podchecker
[dynpmc] adding missing =back in pod found by podchecker
[src] correcting pod errors found by pychecker
[runtime] correcting pod errors found by podchecker
[lib] correcting pod errors found by podchecker
[examples] correcting pod errors found by podchecker
[docs] Correcting pod errors found by podchecker

Removed internal links to the "Upcoming Releases" appendix.  A grep of the
sources shows that this no longer exists within the repository.
[docs] correcting pod errors found by podchecker
[compilers] correcting pod errors found by podchecker
[docs] fix links from ae3e1b74fcb68f65ce20