srcu: Explain the reason behind the read side critical section on GP start

Tell about the need to protect against concurrent updaters who may
overflow the GP counter behind the current update.

Reported-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>