docs/RCU/rcubarrier: Adjust 'Answer' parts of QQs as definition-lists

The "Answer" parts of QQs divert from proper format of definition-lists
as described at [1] and are not rendered as such.

Adjust them.

Link: [1] https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#definition-lists


Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>