Unstable functions can't be collapsed in rustdoc

[tomjakubowski]

This may have been intentional (as to not hide the "unstable" marking when everything is collapsed), but it's still annoying.

I would suggest that the collapsed state could have an indication of the function's instability. Perhaps collapsing a function's description would not hide the unstability marker.

[liigo]

It was fixed by PR #24717 (commit: liigo@76a590d)