doc: Clarify cgroup meta variable

doc: Clarify cgroup meta variable

The documentation mentions control group id where the meaning is a class
id associated to the cgroup of a socket. This used to be fine until
there came cgroup v2 that use similar terminolgy (cgroup id) for very
different thing -- a numeric identifier of a particular (v2) cgroup.

This contemporary cgroup id isn't exposed by netfilter (v2 matching is
based on paths externally). Fix the docs and decrease confusion by more
precise description of the metavariable.

[ Added comment in description to refer to socket cgroupv2 --pablo ]

Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>