nixos/alertmanager-ntfy: improve documentation about topic option (#426956)

              topic = lib.mkOption {

                type = lib.types.str;

                description = ''

                  __Note:__ when using ntfy.sh and other public instances

                  it is recommended to set this option to an empty string and set the actual topic via

                  [](#opt-services.prometheus.alertmanager-ntfy.extraConfigFiles) since

                  the `topic` in `ntfy.sh` is essentially a password.

                  The topic to which alerts should be published.

                  Can either be a hardcoded string or a gval expression that evaluates to a string.

                '';