doc: Fix the description of -n option in lxc-info(1)

Commit 5444216 revised -n option from allowing to specify multiple
containers using regex to specifying only one container. But
lxc-info(1) remains original. so
- mark -n required
- remove the description of -n that is included in common options

Signed-off-by: KATOH Yasufumi <karma@jazz.email.ne.jp>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>