Document internal unsafety

[workingjubilee]

Currently, we document public unsafe fn and their invariants.

However, as a highly unsafe crate, we should also document internally that we are maintaining invariants for unsafe blocks internally. That said, we are also so unsafe because almost everything is a thin wainscotting over extern "platform-intrinsic" calls. Those are not the focus of this issue. Rather, what we really want to document is anything that isn't just a straight pass-through to an internal unsafe fn intrinsic. These comments need not be publically exposed.

A few of these were surfaced in https://github.com/rust-lang/portable-simd/pull/172/files. Mostly, it's things like our internal implementation of existing trait-level unsafe fn. Also we should probably document the safety invariants on the impl too, not just on the trait.

[calebzulawski]

I started implementing this, but found an ICE in clippy: rust-lang/rust-clippy#7934

[calebzulawski]

I think this is basically complete in the feature/unsafety branch, but I'll wait for a fix to the clippy lint so we can keep it up to date with CI.