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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📚 Documentation preview 📚: https://cpython-previews--112399.org.readthedocs.build/

[AlexWaygood]

Thanks!

[AlexWaygood]

LGTM, thanks!

[miss-islington-app]

Thanks @hugovk for the PR 🌮🎉.. I'm working now to backport this PR to: 3.11, 3.12.
🐍🍒⛏🤖

[miss-islington-app]

Sorry, @hugovk, I could not cleanly backport this to 3.11 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 0303a9fa79ab51def97346ae0b9e535de1e33503 3.11

[bedevere-app]

GH-112402 is a backport of this pull request to the 3.12 branch.

[hugovk]

Thanks for the review!

[bedevere-app]

GH-112404 is a backport of this pull request to the 3.11 branch.