workqueue: mark WQ_NON_REENTRANT deprecated

dbf2576e37 ("workqueue: make all workqueues non-reentrant") made
WQ_NON_REENTRANT no-op but the following patches didn't remove the
flag or update the documentation.  Let's mark the flag deprecated and
update the documentation accordingly.

Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index a0ed78a..594521b 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -295,7 +295,12 @@
  * Documentation/workqueue.txt.
  */
 enum {
-	WQ_NON_REENTRANT	= 1 << 0, /* guarantee non-reentrance */
+	/*
+	 * All wqs are now non-reentrant making the following flag
+	 * meaningless.  Will be removed.
+	 */
+	WQ_NON_REENTRANT	= 1 << 0, /* DEPRECATED */
+
 	WQ_UNBOUND		= 1 << 1, /* not bound to any cpu */
 	WQ_FREEZABLE		= 1 << 2, /* freeze during suspend */
 	WQ_MEM_RECLAIM		= 1 << 3, /* may be used for memory reclaim */