Add safety section to `NonNull::as_*` method docs
Add safety section to `NonNull::as_*` method docs

This basically adds the safety section of `*mut T::as_{ref,mut}` to the
same methods on `NonNull` with minor modifications to fit the
differences.

Part of #48929.

aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust