xmllint Return Code 10 (XPath set is empty) not documented

Copy of https://bugzilla.gnome.org/show_bug.cgi?id=778614

The man page for xmllint only lists Return Codes up to and including 9. However, if a valid XPath does not match anything, the message "XPath set is empty" is printed and xmllint terminates with return code 10 (XMLLINT_ERR_XPATH).