Add example for online rolling reboots using tags

While this use case was described in the design document, and
mentioned several times as motivation for changes in commit messages,
it has never been added to a user-facing documentation. This commit
adds at least an example to the man page.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>