rustdoc: "Copy path" button could use a tooltip

[lnicola]

I just noticed the new button, clicked on it, saw the checkmark and had no idea what happened. I thought it reflowed the page somehow, or maybe added the crate to a favorites list :-).

[inquisitivecrystal]

@rustbot label +A-rustdoc-ui +C-enhancement +T-rustdoc

[notriddle]

I think #85859 already fixed this in the current nightly.

[jyn514]

Yup it has, thanks!