doc: Update checklist.rst discussion of callback execution

This commit completes the list of call_rcu*() functions that are not
guaranteed to have their callbacks executing on the same CPU.  While in
the area, fix an unrelated typo.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>