Rollup merge of #142700 - theemathas:remove-weak-comment, r=ibraheemdev

Remove incorrect comments in `Weak`

It is currently possible to create a dangling `Weak` to a DST by calling `Weak::new()` for a sized type, then doing an unsized coercion. Therefore, the comments are wrong.

These comments were added in <rust-lang/rust#73845>. As far as I can tell, the guarantee in the comment was only previously used in the `as_ptr` method. However, the current implementation of `as_ptr` no longer relies on this guarantee.

Author: compiler-errors