Rollup merge of #134619 - hkBst:patch-7, r=jhpratt

Improve prose around `as_slice` example of IterMut

I've removed the cryptic message about not being able to call `&mut self` methods while retaining a shared borrow of the iterator, such as `as_slice` produces. This is just normal borrowing rules and does not seem especially relevant here. I can whip up a replacement if someone thinks it has value.

Author: matthiaskrgr