capabilities: add description for CAP_SETFCAP

Document the purpose of CAP_SETFCAP.  For some reason this capability
had no description while the others did.
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>

include/uapi/linux/capability.h

@@ -331,6 +331,8 @@ struct vfs_ns_cap_data {
 
 #define CAP_AUDIT_CONTROL    30
 
+/* Set or remove capabilities on files */
+
 #define CAP_SETFCAP	     31
 
 /* Override MAC access.