rcu: RCU_TRACE enables event tracing as well as debugfs

The commit brings the RCU_TRACE Kconfig option's help text up to date
by noting that it enables additional event tracing as well as debugfs.

Signed-off-by: Nikolay Borisov <kernel@kyup.com>
[ paulmck:  Do some wordsmithing. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>