Correct errors in WalkDir type docs

[KodrAus]

The docs on WalkDir have a few inaccuracies about the behaviour that should be corrected:

• If contents_first is true, then directories aren't emitted before their contents
• The order isn't unspecified if a sort_by function is given. Or does 'unspecified' mean something different here?

[alisha17]

I can fix this. Where this has to be fixed in the docs?

[KodrAus]

Thanks @alisha17!

See this paragraph in the docs for WalkDir.

They state that directories are always emitted before the contents, and that using sort_by means the items will be emitted according to that ordering.

Let me know if you need any more details.

[BurntSushi]

Fixed by #74