Auto merge of #103413 - RalfJung:phantom-dropck, r=lcnr

PhantomData: fix documentation wrt interaction with dropck

As far as I could find out, the `PhantomData`-dropck interaction *only* affects code using `may_dangle`. The documentation in the standard library has not been updated for 8 years and thus stems from a time when Rust still used "parametric dropck", before [RFC 1238](https://rust-lang.github.io/rfcs/1238-nonparametric-dropck.html). Back then what the docs said was correct, but with `may_dangle` dropck it stopped being entirely accurate and these days, with NLL, it is actively misleading.

Fixes rust-lang/rust#102810
Fixes rust-lang/rust#70841
Cc `@nikomatsakis` I hope what I am saying here is right.^^

Author: bors