add description to the output id argument

After struggling a bit to find the right argument, I think this description is a little more clear to what should be passed (tried the pin number and zero-based array before finding the right way in the code)

API.md

@@ -144,7 +144,7 @@ Error Responses:
 
 Endpoint: **PATCH** `/outputs/<id>`
 
-*Note: id needs to be int (index_id)*
+*Note: id needs to be int (index_id), one-based index in octoprint settings' enclosure section's `rpi_outputs` list*
 
 Body (Content-Type: `application/json`):
 ```
@@ -340,4 +340,4 @@ Body: empty
 Response (204): No-Content
 
 Error Responses:
-- none
+- none