rcu: Make bugginess of code sample more evident

One of the code samples in whatisRCU.txt shows a bug, but someone scanning
the document quickly might mistake it for a valid use of RCU.  Add some
screaming comments to help keep speed-readers on track.

Reported-by: Nathan Zimmer <nzimmer@sgi.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>