Missing docs for `Div`, `Rem`, `Shl` and `Shr` (they show `Mul` instead.)

[Achtuur]

The docs for Div, Rem, Shl and Shr are showing the docs for Mul instead. The culprit is most likely the following:

https://github.com/JelteF/derive_more/blob/3216eaf1cbcd081497add134ddda94a43344ae2d/impl/src/lib.rs#L221-L225

It seems that there are no doc/(div|rem|shl|shr).md, so only changing the $feature string here would not work.