Clarify docstring length in CONTRIBUTING.rst

[tofu-rocketry]

Describe what maintenance you would like added.

This PR #4129 implies that you're settling on a docstring length of 75 to match numpy.

However, your CONTRIBUTING.rst says to follow PEP 8 and 72 char docstrings, but the docstring section refers you to the numpydoc Style Guide and so implicitly endorses 75 characters (as there's no exception from NumPy's line length).

Either the Coding Style section should go with 75 characters, or the Docstring section should list an exception from NumPy's standard.

Links to source code.

Coding Style section: https://github.com/pyvista/pyvista/blob/94c484c6a6a54b86704168a54666323234d7fdf5/CONTRIBUTING.rst#coding-style

Docstring section: https://github.com/pyvista/pyvista/blob/94c484c6a6a54b86704168a54666323234d7fdf5/CONTRIBUTING.rst#docstrings

Pseudocode or Screenshots

No response