Document `// SAFETY:` comments.

[m-ou-se]

We should document what we expect from // Safety: .. comments.

See also rust-lang/rust#66219

[poliorcetics]

Where would this go in the dev guide ? I can write something but I don't know the guide well enough to have an idea of where I should put it.

[JohnTitor]

I think under https://std-dev-guide.rust-lang.org/documentation/summary.html would be fine.