Problem

The docstring of legend() currently states

        Call signatures::

            legend()
            legend(labels)
            legend(handles, labels)

so the signature legend(handles=handles) is missing (it is explicitly listed in _parse_legend_args). I think that signature is actually the second most important one (after legend()) given that it's the applicable one in a pure OO setting (where labels are directly set on the artists), so it should be added and the rest of the docstring amended accordingly.

Suggested Improvement

See above.

Matplotlib version

• Operating system: linux
• Matplotlib version (import matplotlib; print(matplotlib.__version__)): HEAD (3.4.x)
• Matplotlib documentation version (is listed under the logo): 3.4.x