Should be something like

        Request a widget redraw once control returns to the GUI event loop.

        Even if multiple calls to `draw_idle` occur before control
        returns to the GUI event loop, (backends should arrange that)
        the figure will only be rendered once.

where the parenthesis depends on whether you're writing the docs for backend implementers or for end users.

Thanks. IMO the main audience of the docs are the end users. I've dropped the parenthesis but left my note instead. That keeps the information in but a bit separated.

Btw. can we make a suggestion to the end-user if she should call draw() or draw_idle()? We have both cases quite mixed in the examples.

If the draw event happens inside the event loop one should definitely call draw_idle(). Actually I think my very first PR here was about exactly that (#9064). If no event loop has been started, draw_idle() potentially has no effect.
There is also #5800, which suggests there actually is a drawback from calling it with non-GUI backends.