srcu: Document auto-expediting requirement

This commit documents the auto-expediting requirement satisfied by
commits 2da4b2a7 ("srcu: Expedite first synchronize_srcu() when idle")
and 22607d66 ("srcu: Specify auto-expedite holdoff time").

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