Closed
Description
It should be mentioned in the docstring (https://github.com/matplotlib/matplotlib/blob/master/lib/matplotlib/figure.py#L1973), even though the allowable keys depend on the target format.
It was introduced in https://matplotlib.org/devdocs/users/prev_whats_new/whats_new_2.1.0.html#metadata-savefig-keyword-argument.