Auto merge of #43614 - pornel:is_gotcha, r=BurntSushi

Auto merge of #43614 - pornel:is_gotcha, r=BurntSushi

Emphasise that these functions look at the disk, not just the path

I thought that `PathBuf::new("foo/bar/").is_dir()` is always true, because the path ends in `/`. However, this is not what `is_dir()` function does. So I've updated the docs to make it clear.