Doc: add some precisions for --remote and dumb mode

Doc: add some precisions for --remote and dumb mode

The -r/--remote argument to some of bup's commands currently doesn't
give enough information about how to customize options to SSH. Let's add
information about this so that users know how to customize options for
SSH connections.

Also, in bup-server's documentation, point out which mode is the default
one for more clarity.

Signed-off-by: Gabriel Filion <lelutin@gmail.com>