docs cgroups: add another example size for hugetlb

Add another example to clarify that HugePages smaller than 1MB will
be displayed using "KB", with an uppercased K (eg. 20KB), and not the
normal SI prefix kilo (small k).

Because of a misunderstanding/copy-paste error inside runc
(see https://github.com/opencontainers/runc/pull/2065), it tried
accessing the cgroup control file of a 64kB HugePage using
"hugetlb.64kB._____" instead of the correct "hugetlb.64KB._____".

Adding a new example will make it clear how sizes smaller than 1MB are
handled.

Signed-off-by: Odin Ugedal <odin@ugedal.com>
Signed-off-by: Tejun Heo <tj@kernel.org>