srcu: Document that srcu_funnel_gp_start() implies srcu_funnel_exp_start()

This commit updates the header comment of srcu_funnel_gp_start() to
document the fact that srcu_funnel_gp_start() does the work of
srcu_funnel_exp_start(), in some cases by invoking it directly.

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