docs: cgroup-v1: replace custom note constructs with appropriate admonition blocks

Admonition constructs on the documentation use definition lists, which
isn't fit for the purpose. Replace them with appropriate blocks:

  * Use caution:: for outdated document notice
  * hint:: for memo
  * note:: for other constructs
  * warning:: for memory reclaim

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>