Rollup merge of #113510 - ink-feather-org:const_ptr_transmute_docs, r=RalfJung

Document soundness of Integer -> Pointer -> Integer conversions in `const` contexts.

see this [zulip thread](https://rust-lang.zulipchat.com/#narrow/stream/146212-t-compiler.2Fconst-eval/topic/Soundness.20of.20Integer.20-.3E.20Pointer.20-.3E.20Integer.20conversions)

r? `@RalfJung`

With this slice `Iterator`'s should be able to be made const once the const Trait reimplementation is done.

Author: matthiaskrgr