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 https://github.com/rust-lang/rust/issues/102810 Fixes https://github.com/rust-lang/rust/issues/70841 Cc `@nikomatsakis` I hope what I am saying here is right.^^