DOC: Clarify y parameter in Axes.set_title

[StefRe]

PR Summary

Autopositioning only works if rcParams["axes.titley"] is None in addition
to y being None in the function call. If rcParams["axes.titley"] is set
to a float, there's no way to enforce autopositioning by function call
parameters (passing y=None has no effect in this case).

Link to current doc: https://matplotlib.org/devdocs/api/_as_gen/matplotlib.axes.Axes.set_title.html

Example:

import matplotlib.pyplot as plt

plt.rcParams["axes.titley"] = 1.0
fig, ax = plt.subplots()
ax.xaxis.tick_top()
ax.set_title('Title', y=None)  # trying to override rcParams["axes.titley"] here

PR Checklist

Tests and Styling

• [N/A] Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• [N/A] New features are documented, with examples if plot related.
• [N/A] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• [N/A] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).
• Documentation is sphinx and numpydoc compliant (the docs should build without error).