Rollup merge of #136775 - robertbastian:patch-2, r=Amanieu

Update `String::from_raw_parts` safety requirements

These have become out of sync with `Vec::from_raw_part`'s safety requirements, and are likely to diverge again. I think it's safest to just point at `Vec`'s requirements.

rust-lang/rust#119206 (comment)

Author: tgross35