Update Matplotlib docs (#14425)...
Update Matplotlib docs (#14425)
This updates the docs in line with the recent changes to move Matplotlib
backend resolution to Matplotlib itself. In passing I have removed
references to things that have definitely disappeared from Matplotlib
(such as `qt4`), and I've added a comment about when
`_matplotlib_manages_backends()` can be removed in future.
This, along with
#14420, should conclude the backend transition.
