rcu: Improve SRCU's grace-period comments
rcu: Improve SRCU's grace-period comments

This commit documents the memory-barrier guarantees provided by
synchronize_srcu() and call_srcu().

Signed-off-by: Paul E. McKenney <1e0ce936bb9b355d257bf5790d2513c3f28be22b@linux.vnet.ibm.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs