add documentation for "?" file types
add documentation for "?" file types

Summary:
we can technically return a "?" if we observe a whiteout entry. "Whiteout directory entries don't refer to any real inode, but for easy compatibility with existing file system utilities such as fsck, each whiteout directory entry includes a faux inode number". This can occur in union filesystems and in some cases may actually leak into user space.

see http://sourceware-org.1504.n7.nabble.com/BZ-6895-Describe-DT-LNK-td5495.html for more information

requested here: https://github.com/facebook/watchman/issues/892

Reviewed By: chadaustin

Differential Revision: D26466440

fbshipit-source-id: 6a00c6b5e3de99b98ff4435edf1e783655859b42

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