Rollup merge of #58033 - euclio:rustdoc-tags, r=QuietMisdreavus

rustdoc: wrap stability tags in colored spans

A cosmetic change to make the stability tags stand out a bit against the docs. Opening for discussion.

Before:

![screen shot 2019-01-31 at 3 29 36 pm](https://user-images.githubusercontent.com/1372438/52083406-54730d80-256d-11e9-8e61-b8caff569434.png)
![screen shot 2019-01-31 at 3 31 32 pm](https://user-images.githubusercontent.com/1372438/52083408-54730d80-256d-11e9-97b7-43e808448f65.png)

After:
![screen shot 2019-01-31 at 3 29 18 pm](https://user-images.githubusercontent.com/1372438/52083405-54730d80-256d-11e9-9983-19d9519b2ed8.png)
![screen shot 2019-01-31 at 3 29 46 pm](https://user-images.githubusercontent.com/1372438/52083407-54730d80-256d-11e9-8c32-11a1ad7d3f34.png)

r? @QuietMisdreavus

diff --cc src/librustdoc/html/render.rs
Simple merge