Add more info about deleted forms to API docs

[ktuite]

Problem description

It's not clear in the API docs where the numeric ID of form comes from. This is used for restoring a form. That part of the docs says the ID comes from the /forms endpoint, but that is only true for deleted forms. I don't see anything in the docs currently about listing (soft)-deleted forms.

This came up in the forum and is relevant to any new work we do on deleting in the future.

• Restoring section should more clearly describe .../forms?deleted=true
• Form list section should probably describe deleted forms.