Clean up docs and refactor createhandler

**What**
- Add a description for the secret key to the api swagger spec.
- Remove optional examples from the secret management guide.
- Update the ApiKeyProtected README to point at the new guide.
- Refactor the `makeSpec` function to accept the already assembled secrets
array because this should be easier to unit test.

Signed-off-by: Lucas Roesler <lucas.roesler@gmail.com>

api-docs/swagger.yml

@@ -229,6 +229,7 @@ definitions:
         type: array
         items:
           type: string
+        description: An array of names of secrets that are required to be loaded from the Docker Swarm.
       registryAuth:
         type: string
         description: >-