Social Notes: clarify usage of WP.me shortlink for shortlink and permashortcitation formats

[jeherve]

Social Notes offers the option to use a shortlink format or a permashortcitation format. When you do that, it uses a WP.me shortlink.

While this makes sense when one uses the Jetpack plugin and has enabled the Shortlinks feature there, I think it would make sense to clarify things when one either does not use Jetpack, or does not use the Shortlinks feature there.

• We could update the UI to clarify that this uses WP.me shortlinks, and not core WordPress shortlinks.

• We could respect the shortlink module status when the Jetpack plugin is installed and the module is disabled.