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

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.

The issue names no files, tests, or entry points. Start by locating the Social Notes settings UI and the Jetpack Shortlinks module handling, then clarify which proposed behavior is intended; completion should make the WP.me usage clear and/or respect a disabled Shortlinks module.

Written by the indexing model from the issue text.