bpo-36064: Clarify allowed data types for urllib.request.Request. (GH-11990)

Doc/library/urllib.request.rst

@@ -192,8 +192,8 @@ The following classes are provided:
    *data* must be an object specifying additional data to send to the
    server, or ``None`` if no such data is needed.  Currently HTTP
    requests are the only ones that use *data*.  The supported object
-   types include bytes, file-like objects, and iterables.  If no
-   ``Content-Length`` nor ``Transfer-Encoding`` header field
+   types include bytes, file-like objects, and iterables of bytes-like objects.
+   If no ``Content-Length`` nor ``Transfer-Encoding`` header field
    has been provided, :class:`HTTPHandler` will set these headers according
    to the type of *data*.  ``Content-Length`` will be used to send
    bytes objects, while ``Transfer-Encoding: chunked`` as specified in