PR Summary

This updates the ax.acorr and ax.xcorr documentation and code to reflect the numpy documentation. Specifically, the code currently calls np.correlate with mode=2, while the documentation in numpy for mode reads:

mode : {'valid', 'same', 'full'}, optional
    Refer to the `convolve` docstring.  Note that the default
    is 'valid', unlike `convolve`, which uses 'full'.

It turns out mode=2 eventually maps to mode="full", and this change makes that explicit.

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way