doc: rcu: Update description of gp_seq fields in rcu_data

The rcu_state structure doesn't have a gp_seq_needed field. Update the
description under rcu_data accordingly, to reflect this.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: <kernel-team@android.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>