gh-101100 : Fix Sphinx warnings in library/doctest.rst

[hugovk]

Fixes 24 Sphinx warnings:

❯ SPHINXERRORHANDLING=-n PATH=./venv/bin:$PATH sphinx-build -b html -d build/doctrees -j auto -n . build/html library/doctest.rst 2>&1 | grep doctest.rst | tee >(wc -l)
Doc/library/doctest.rst:145: WARNING: py:mod reference target not found: M
Doc/library/doctest.rst:152: WARNING: py:mod reference target not found: M
Doc/library/doctest.rst:405: WARNING: py:mod reference target not found: M
Doc/library/doctest.rst:405: WARNING: py:mod reference target not found: M
Doc/library/doctest.rst:405: WARNING: py:mod reference target not found: M
Doc/library/doctest.rst:959: WARNING: py:meth reference target not found: doctest.master.summarize
Doc/library/doctest.rst:996: WARNING: py:func reference target not found: load_tests
Doc/library/doctest.rst:1019: WARNING: py:exc reference target not found: failureException
Doc/library/doctest.rst:1085: WARNING: py:exc reference target not found: failureException
Doc/library/doctest.rst:1115: WARNING: py:class reference target not found: doctest.DocTestCase
Doc/library/doctest.rst:1115: WARNING: py:class reference target not found: DocTestCase
Doc/library/doctest.rst:1115: WARNING: py:class reference target not found: DocTestCase
Doc/library/doctest.rst:1121: WARNING: py:class reference target not found: doctest.DocFileCase
Doc/library/doctest.rst:1121: WARNING: py:class reference target not found: DocFileCase
Doc/library/doctest.rst:1121: WARNING: py:class reference target not found: DocTestCase
Doc/library/doctest.rst:1125: WARNING: py:class reference target not found: DocTestCase
Doc/library/doctest.rst:1146: WARNING: py:meth reference target not found: runTest
Doc/library/doctest.rst:1146: WARNING: py:class reference target not found: DocTestCase
Doc/library/doctest.rst:1146: WARNING: py:class reference target not found: DocTestCase
Doc/library/doctest.rst:1146: WARNING: py:class reference target not found: DocTestCase
Doc/library/doctest.rst:1321: WARNING: py:attr reference target not found: optionflags
Doc/library/doctest.rst:1561: WARNING: py:meth reference target not found: DocTestRunner.report_*
Doc/library/doctest.rst:1733: WARNING: py:func reference target not found: f
Doc/library/doctest.rst:1743: WARNING: py:func reference target not found: f
      24

M and f are example code, skip 'em.

These docs explicitly say "DocTestCase isn’t documented here (it’s an internal detail)" so skip it and its method too, and also the similar DocFileCase.

Add docs for the failureException exception.

• Issue: Fix all Sphinx reference warnings in the documentation #101100

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