doc: note meta cgroup returns zero on cgroupv2-only hosts

doc: note meta cgroup returns zero on cgroupv2-only hosts

Commit 2ff29969 ("doc: Clarify cgroup meta variable") corrected the
terminology and pointed readers to socket cgroupv2. The man page still
gives no indication that meta cgroup silently returns zero on
cgroupv2-only hosts with CONFIG_CGROUP_NET_CLASSID=y (the distribution
default) and no active net_cls hierarchy, the common configuration
on modern systems. Rules load without error and match nothing.

Make the behaviour explicit in the meta expression types table.

Signed-off-by: Avinash H. Duduskar <avinashhd@protonmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>