rcu/tree: docs: document bkvcache new members at struct kfree_rcu_cpu

Changeset 53c72b59 ("rcu/tree: cache specified number of objects")
added new members for struct kfree_rcu_cpu, but didn't add the
corresponding at the kernel-doc markup, as repoted when doing
"make htmldocs":
	./kernel/rcu/tree.c:3113: warning: Function parameter or member 'bkvcache' not described in 'kfree_rcu_cpu'
	./kernel/rcu/tree.c:3113: warning: Function parameter or member 'nr_bkv_objs' not described in 'kfree_rcu_cpu'

So, move the description for bkvcache to kernel-doc, and add a
description for nr_bkv_objs.

Fixes: 53c72b59 ("rcu/tree: cache specified number of objects")
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>