Rollup merge of #109540 - marcospb19:edit-Path-with_file_name-example, r=m-ou-se

std docs: edit `PathBuf::set_file_name` example

To make explicit that `set_file_name` might replace or remove the
extension, not just the file stem.

Also edit docs for `Path::with_file_name`, which calls `set_file_name`.

Author: matthiaskrgr