Rollup merge of #89538 - notriddle:notriddle/arrow-highlight, r=GuillaumeGomez

Make rustdoc not highlight `->` and `=>` as operators

It was marking them up as `<span class="op">=</span><span class="op">&gt;</span>`,
which is bloaty and wrong (at least, I think `<=` and `=>` should probably be different colors, since they're so different and yet made from the same symbols).

Before:

![image](https://user-images.githubusercontent.com/1593513/135939748-f49b0f9e-6a7d-4d65-935a-e31cdf688a81.png)

After:

![image](https://user-images.githubusercontent.com/1593513/135940063-5ef1f6b1-7e03-4227-b46b-572b063aba05.png)