(fs spec) Re-visit Path.copyTo/moveTo spec for case that target exists and is non-empty directory

As noted on the nio-discuss mailing lists, the moveTo method specifies that FileAlreadyExistsException is thrown if:

"the target file exists and cannot be replaced because the REPLACE_EXISTING option is not specified, or the target file is a non-empty directory"

The second part covers the "source.moveTo(target, REPLACE_EXISTING)" case and the target is a directory that cannot be replaced because it contains files. This statement should be re-examined. It would be more logical to list DirectoryNotEmptyException as an optional exception for this case. The copyTo method has the same issue.