figure.max_num_figures was renamed to figure.max_open_warning.
Merge pull request #3640 from emikulic/master

DOC : figure.max_num_figures was renamed to figure.max_open_warning.

This was done in 8c8bc8f71aa373b076d8790e14046c940a5269e0 and 0433bd59e43e63ea792a970c32a883e4aaa3d086