Fix pod errors

These changes correct almost all of the pod errors found by `podchecker`.

The only file I can't update is `lib/IO/CaptureOutput.pm` since this file contains embedded wikidoc documentation, which is similar to POD and thus causes `podchecker` to raise errors.  Should I convert this documentation to POD?  Then the `pod_syntax.t` test would pass.

Comments as usual welcome.