Fix various Sphinx warnings #4787

Job	Run time
clang-format	23s
copyright_headers	15s
cppcheck	4m 15s
rstcheck	29s
vale	39s
forbidden_types	20s
lychee_IGNORED	24s
mypy	36s
pydocstyle	19s
pylint	33s
unused_names	18s
black	27s
flake8	26s
isort	17s
build_macos (macos-latest, clang, openmp, mpi, python, gsl, ltdl, boost, hdf5, optimize, warning)	31m 29s
build_linux (ubuntu-20.04, gcc, boost, optimize, warning)	3m 42s
build_linux (ubuntu-20.04, gcc, openmp, python, gsl, ltdl, boost, optimize, warning)	17m 49s
build_linux (ubuntu-20.04, gcc, mpi, python, gsl, ltdl, boost, optimize, warning)	18m 40s
build_linux (ubuntu-20.04, gcc, openmp, mpi, python, gsl, ltdl, boost, hdf5, sionlib, libneurosim...	24m 19s
sphinx-rtd	3m 26s
sphinx-conda	0s
	1h 49m 6s

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