Clarify that PreparedRequests should not be manually created
Closes
@@ -273,7 +273,9 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
"""The fully mutable :class:`PreparedRequest <PreparedRequest>` object,
containing the exact bytes that will be sent to the server.
- Generated from either a :class:`Request <Request>` object or manually.
+ Instances are generated from a :class:`Request <Request>` object, and
+ should not be instantiated manually; doing so may produce undesirable
+ effects.
Usage::
