workqueue: mark WQ_NON_REENTRANT deprecated

dbf2576e ("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>