Documentation: Record bottom-bit-zero guarantee for ->next

This commit records RCU's guarantee that the bottom bit of the rcu_head
structure's ->next field will remain zero for callbacks posted via
call_rcu(), but not necessarily for <tt>kfree_rcu()</tt> or some
possible future call_rcu_lazy() variant that might one day be created
for energy-efficiency purposese.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Updates URLs as suggested by Josh Triplett. ]