%PYTHONPATH% not set in python/docs/make2.bat, resulting in failed/wrong documentation builds

When building the PySpark documentation on Windows, by changing directory to python/docs and running make.bat (which runs make2.bat), the majority of the documentation may not be built if pyspark is not in the default %PYTHONPATH%. Sphinx then reports that pyspark (and possibly dependencies) cannot be imported.

If pyspark is in the default %PYTHONPATH%, I suppose it is that version of pyspark – as opposed to the version found above the python/docs directory – that is considered when building the documentation, which may result in documentation that does not correspond to the development version one is trying to build.

python/docs/Makefile avoids this issue by setting
export PYTHONPATH=$(realpath ..):$(realpath ../lib/py4j-0.10.8.1-src.zip)
on line 10, but make2.bat does no such thing. The fix consist of adding
set PYTHONPATH=..;..\lib\py4j-0.10.8.1-src.zip
to make2.bat.

See GitHub PR #27569.