docs: maintainers: mention tag -s for signed tags

The documentation talks about -u and how to configure the default
key. It does not mention that once the default key is set one
should use the -s flag. Which is likely what most people end up
using.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20230713230606.1505458-1-kuba@kernel.org>