Make the build more strict on documentation errors

* Detect undefined labels. This can be tested by adding :ref:`nonexistent`
  to a documentation rst file; attempting to build docs will fail.
  Fixed the undefined label in `9.8.1-notes.rst`.
* Detect errors. While we have plenty of warnings, we can at least enforce
  that Sphinx does not report errors.
  Fixed the error in `required_type_arguments.rst`.

Unrelated change: I have documented that the `-dlint` enables
`-fcatch-nonexhaustive-cases`, as can be verified by checking
`enableDLint`.