Rollup merge of #141946 - xizheyin:141933, r=jhpratt

std: refactor explanation of `NonNull`

Fixes rust-lang/rust#141933

I cut out the excessive explanation and used an example to explain how to maintain invariance, but I think what is quoted in the *rust reference* in the document needs to be added with a more layman's explanation and example.

(I'm not sure if I deleted too much)

r? `@workingjubilee`

Author: workingjubilee