rustdoc: Show nag box on IE10
rustdoc: Show nag box on IE10

Rustdoc doesn't work on IE11. It's been broken for months, it isn't supported
by the [tiered browser support list], it's even more severely broken on other
Rust websites, and IE11 doesn't support the `<details>` tag that we want
to use.

In the interest of honesty, let's give an actual error message for anyone
on IE11.

[tiered browser support list]: https://github.com/rust-lang/rfcs/blob/master/text/1985-tiered-browser-support.md

graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust