net: sched: Add description for cpu_bstats argument

Commit 22e0f8b9 ("net: sched: make bstats per cpu and estimator RCU safe")
added the argument cpu_bstats to functions gen_new_estimator and
gen_replace_estimator and now the descriptions of these are missing for the
documentation. Adding them.

Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>