Add docs for --dns-search=.
Add docs for --dns-search=.

PR 6720 introduce that use `--dns-search=.` will not set `search` in `/etc/resolv.conf`.

Signed-off-by: Huayi Zhang <7c9694756c124751081929d2be5b5bc5b14fe9b7@gmail.com>

Zebrilee/cli,Zebrilee/cli,dnephin/cli,cpuguy83/cli,ashfall/cli,ashfall/cli,thaJeztah/cli,cpuguy83/cli,redpanda/cli,e11137/cli,e11137/cli,docker/cli,riyazdf/cli,thaJeztah/cli,dnephin/cli,riyazdf/cli,allencloud/cli,redpanda/cli,allencloud/cli,docker/cli