docs/bpf: Add documentation for new cgroup local storage

Add some descriptions and examples for BPF_MAP_TYPE_CGRP_STORAGE.
Also illustate the major difference between BPF_MAP_TYPE_CGRP_STORAGE
and BPF_MAP_TYPE_CGROUP_STORAGE and recommend to use
BPF_MAP_TYPE_CGRP_STORAGE instead of BPF_MAP_TYPE_CGROUP_STORAGE
in the end.

Acked-by: David Vernet <void@manifault.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221026042922.676383-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>