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 <irachex@gmail.com>
Mapping change in code

Signed-off-by: Huayi Zhang <irachex@gmail.com>
Merge pull request #8839 from irachex/dns-search-doc

Add docs for --dns-search=.