Sphinx CI job fails on Sphinx v2.4.0

[espresso-ci]

https://gitlab.icp.uni-stuttgart.de/espressomd/espresso/pipelines/11044

[jngrad]

I'm investigating. cuda:10.1 was rolled back to an earlier version for the time being.

[jngrad]

In our CMake logic, we have:

espresso/doc/sphinx/CMakeLists.txt

Lines 74 to 79 in 55375b8

	${SPHINX_API_DOC_EXE}
	-f
	-P
	-o ${SPHINX_BASE}
	${SPHINX_PYTHON_DIR}
	${EXCLUDE}

where the -P flag means documenting private members, i.e. all classes and methods starting with a single underscore symbol. This flag was introduced in d8c56d7, a few days after the private-members option was added in c5e9fa2. However, the -P never had any effect due to a bug that only got fixed recently by Sphinx 2.4.0. When we decided to stop documenting the private members in #1296, the private-members option was removed in 49262af but not the -P flag.

Now that cuda:10.1 has Sphinx 2.4.0, the private members are processed to generate the HTML output, revealing multiple errors in the docstrings. I'll fix those errors and remove the -P flag to hide private members again.