Clarify behaviour of `json` parameter.
`json` is ignored if `data` or `files` is not empty.
@@ -281,6 +281,7 @@ the ``json`` parameter (added in version 2.4.2) and it will be encoded automatic
>>> r = requests.post(url, json=payload)
+Note, the ``json`` parameter is ignored if either ``data`` or ``files`` is passed.
POST a Multipart-Encoded File
-----------------------------
