From thread on help list does tidy cross file system boundary?
The description in the documentation for xdev currently says true/false exclude directories that are on different devices and the attribute.
This can easily (but incorrectly) be understood as indicating that when xdev is true, it allows you to cross file system boundaries. In fact, it's the opposite, when xdev is true, files and directories on different devices (file system boundaries) are excluded.
The wording should be clarified to make misinterpretation less likely.