Apply small fixes to (mostly) docstrings of `config_parser` module and extend unit test

Our possibly best documentation of the expected pipeline configuration format is the module's docstring, which contains several small mistakes and is outdated or incomplete in several places (see https://icecube.github.io/pisa/pisa.utils.html#module-pisa.utils.config_parser).

There also seems to be the same small bug in two methods of PISAConfigParser:

I could not find out whether the _validate_value_types method of RawConfigParser ever returned something. At the time the code was introduced, https://github.com/icecube/pisa/blob/master/pisa/utils/config_parser.py#L1184 suggests it was based on https://pypi.org/project/configparser/3.5.0/.
In all recent versions (for example, https://github.com/python/cpython/blob/3.10/Lib/configparser.py#L1171), _validate_value_types does not return anything. In any case, the two methods are now identical to the same ones of ConfigParser (https://github.com/python/cpython/blob/3.10/Lib/configparser.py#L1202 and https://github.com/python/cpython/blob/3.10/Lib/configparser.py#L1208).
The only place I could find where an affected method is used is in https://github.com/icecube/pisa/blob/44ac13df4dbea3c0b12068cbd1c44234cb6f5f1d/pisa/core/pipeline.py#L738, but that script is already broken in various other ways -> now included in unit test

icecube / pisa