How can I generate a full swagger doc for Magento 2 REST API?

That's actually correct behavior.

You need to generate API credentials in magento backend, authenticate, and insert token like on the image below:

Token can be obtained from swagger web interface calling integrationAdminTokenServiceV1 or you can authenticate use curl:

curl -XPOST -H 'Content-Type: application/json' http://your-host/index.php/rest/V1/integration/admin/token -d '{ "username": "yourUsername", "password": "yourPassword" }'

To see the full list of services you may want to generate a full permissions token from the admin panel.

1. log into admin panel
2. Navigate to Systems -> Integrations (under Extensions section)
3. click on "Add a new Integration" Button
4. Fill in the required fields and move to API tab on the left, for "Resource Access" select All
5. Hit save and in the grid view click on Authorize.
6. Grab the Access Token generated (and save the rest of the info, of course).
7. Navigate to /swagger and paste that access token to see the full list of REST services on your site.