Auto merge of #130689 - RalfJung:rustc_nonnull_optimization_guaranteed, r=jieyouxu

fix rustc_nonnull_optimization_guaranteed docs

As far as I can tell, even back when this was [added](rust-lang/rust#60300) it never *enabled* any optimizations. It just indicates that the FFI compat lint should accept those types for NPO.

Author: bors