doc: document lxc.seccomp.notify.cookie
doc: document lxc.seccomp.notify.cookie

and fix a minor typo

Signed-off-by: Wolfgang Bumiller <bac7898f539fd233d9ce33be7a9eda13ee647930@proxmox.com>

hallyn/lxc,hallyn/lxc,hallyn/lxc