parsers: clarify documentation of test-parseindex2.py
This change updates and improves the description of test-parseindex2.py.
In particular, it removes language that can be interpreted to mean that the
test module checks only the C implementation of parsers.parse_index2().
Rather, the module checks parsers.parse_index2(), which can be either the
C or pure Python implementation, depending on which version is being used.
As of e57c532c3835, the module also does more than just compare the return
value with the original Python implementation.