Clean up docstrings and function signatures after #595

[alvii147]

A few non-functionality-based things need to be cleaned up after #595:

• k should be removed as a parameter from here as it is not used anywhere.
• PL and PR need not be assigned here as they are not used after calling _stump
• Docstrings here and here, and the comments in here should use g instead of k
• Numba function signatures should be updated here and here