I found the hierarchy mode useful, but it's easy to make a typo when
using it.  Let's add a short option for that.

Also update the documentation. :)
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Link: https://lore.kernel.org/r/20240125055124.1579617-1-namhyung@kernel.orgSigned-off-by: Namhyung Kim <namhyung@kernel.org>