Add safety documentation

[aumetra]

Closes #2305

I hope this documentation makes it a little clearer why the usage here is sound.
Best proof-read by someone that isn't me to make sure what I wrote is actually coherent for others.

[aumetra]

I guess the part where we say "pointer is required to be non-null can be covered by simply utilizing the std::ptr::NonNull type. Also opens up some fun Option optimizations (not that we really need them) where Option<Region> == Region since they can encode the Option tag as ptr == ptr::null()

[chipshort]

simply utilizing the std::ptr::NonNull type

That sounds nice. I'm always a fan of enforcing correct usage through the type system where possible.

[aumetra]

(had to fix up the Changelog a bit)

[chipshort]

LGTM