bpftool: Add sock_release help info for cgroup attach/prog load command

The help information was not added at the time when the function got added.
Fix this and add the missing information to its cli, documentation and bash
completion.

Fixes: db94cc0b ("bpftool: Add support for BPF_CGROUP_INET_SOCK_RELEASE")
Signed-off-by: Liu Jian <liujian56@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20210525014139.323859-1-liujian56@huawei.com