Document lang items, #[no_std], and runtimeless Rust

[bstrie]

Sub-bug of #3608.

[thestinger]

If we do add documentation for these, we need to make it very clear that they are internal implementation details and will never feasibly have backwards compatibility guarantees.

[steveklabnik]

@thestinger is that still true, nine months later? Just curious. runtimeless-rust should have so few dependencies, I'd imagine. Is keeping backwards compatibility here hard?

[steveklabnik]

"Using Rust without a Runtime" should be a guide, for sure.

[thestinger]

The basics are now documented in the unsafe guide. The lang items and intrinsics are ever changing implementation details so documenting it will actually be worse than having people look in crates like liballoc.