Rollup merge of #95547 - RalfJung:ptr-int-transmutes, r=scottmcm

caution against ptr-to-int transmutes

I don't know how strong of a statement we want to make here, but I am very concerned that the current docs could be interpreted as saying that ptr-to-int transmutes are just as okay as transmuting `*mut T` into an `&mut T`.

Examples [like this](https://github.com/rust-lang/unsafe-code-guidelines/issues/286#issuecomment-1085144431) show that ptr-to-int transmutes are deeply suspicious -- they are either UB, or they don't round-trip properly, or we have to basically say that `transmute` will actively look for pointers and do all the things a ptr-to-int cast does (which includes a global side-effect of marking the pointed-to allocation as 'exposed').

Another alternative might be to simply not talk about them... but we *do* want people to use casts rather than transmutes for this.

Cc `@rust-lang/lang`

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