Document the slow work thread pool

Document the slow work thread pool.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Steve Dickson <steved@redhat.com>
Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Tested-by: Daire Byrne <Daire.Byrne@framestore.com>
diff --git a/kernel/slow-work.c b/kernel/slow-work.c
index 3f65900..cf2bc01 100644
--- a/kernel/slow-work.c
+++ b/kernel/slow-work.c
@@ -7,6 +7,8 @@
  * modify it under the terms of the GNU General Public Licence
  * as published by the Free Software Foundation; either version
  * 2 of the Licence, or (at your option) any later version.
+ *
+ * See Documentation/slow-work.txt
  */
 
 #include <linux/module.h>