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

* Remove `prefix` param of `document_short/full`, `render_markdown`, as its always an empty string.
* Remove `Option` wrapping of `document_short` `parent`, as its always `Some`.

Clarify the difference between insert and get_or_insert

Took me a while to realize that the two are actually different.

rustdoc: Show nag box on IE11

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

…ark-Simulacrum

Remove comment about doc hack.

I neglected to remove this comment in rust-lang#83955.

…yn514

rustdoc: Simplify some document functions

* Remove `prefix` param of `document_short/full`, `render_markdown`, as its always an empty string.
* Remove `Option` wrapping of `document_short` `parent`, as its always `Some`.