Rollup merge of #142236 - yotamofek:pr/std/pathbuf-extend-docs, r=tgross35

Add documentation for `PathBuf`'s `FromIterator` and `Extend` impls

I think it's not very obvious that `PathBuf`'s `Extend` and `FromIterator` impls work like `PathBuf::push`, so I think these should be documented.
I'm not very happy with the wording and examples, open to suggestions :)

Author: Kobzol