File Shadowing is not available on MS Windows

File Shadowing is not available on MS Windows

* doc/emacs/files.texi (File Shadowing): File Shadowing is not
available on MS Windows.

* test/lisp/shadowfile-tests.el (shadow-test00-clusters)
(shadow-test01-sites, shadow-test02-files)
(shadow-test03-expand-cluster-in-file-name)
(shadow-test04-contract-file-name, shadow-test05-file-match)
(shadow-test06-literal-groups, shadow-test07-regexp-groups)
(shadow-test08-shadow-todo, shadow-test09-shadow-copy-files):
Skip under MS Windows.