Postal (docs): Clarify POSTAL_API_URL value

POSTAL_API_URL should be the base URL without API paths, as Anymail appends these automatically.

Author: edgarsj

docs/esps/postal.rst

@@ -46,7 +46,15 @@ nor ``ANYMAIL_POSTAL_API_KEY`` is set.
 
 .. rubric:: POSTAL_API_URL
 
-Required. The base url for calling the Postal API.
+Required. The base URL of your Postal server (without /api/v1 or any API paths).
+Anymail will automatically append the required API paths.
+
+  .. code-block:: python
+
+      ANYMAIL = {
+          ...
+          "POSTAL_API_URL": "https://yourpostal.example.com",
+      }
 
 
 .. setting:: ANYMAIL_POSTAL_WEBHOOK_KEY