rustdoc: Display collapse button on mobile

[ishitatsuyuki]

The collapse button is extremely useful for briefing, and as #45502 is adding the second button, we should show the +/- button as well.

cc @GuillaumeGomez

[GuillaumeGomez]

The +/- buttons are there as well. Which ones are you talking about?

[ishitatsuyuki]

Uh, I mean the master "collapse all" button.

[GuillaumeGomez]

Oh I see. I'll give it a look.

[GuillaumeGomez]

The master "collapse all" button is already displayed as well (on nightly). I'll close this issue then!