docs: Update limitations doc for --cpus
docs: Update limitations doc for --cpus

Mention that although `--cpus` is not supported, `--cpu-quota` and `--cpu-period` are [1].

Fixes #494.

[1] - See https://github.com/containers/virtcontainers/pull/344 and
https://github.com/clearcontainers/runtime/issues/436.

Signed-off-by: James O. D. Hunt <348fbaadfbada55ea105602bf5609e23ac1c4898@intel.com>

devimc/runtime,chavafg/runtime,chavafg/runtime,devimc/runtime,clearcontainers/runtime,clearcontainers/runtime