merge from 3.2. Issue #13073 - Address the review comments made by Ezio.

Doc/library/http.client.rst

@@ -475,11 +475,10 @@ also send your request step by step, by using the four functions below.
 .. method:: HTTPConnection.endheaders(message_body=None)
 
    Send a blank line to the server, signalling the end of the headers. The
-   optional message_body argument can be used to pass message body
-   associated with the request.  The message body will be sent in
-   the same packet as the message headers if possible.  The
-   message_body should be a string.
-
+   optional *message_body* argument can be used to pass a message body
+   associated with the request.  The message body will be sent in the same
+   packet as the message headers if it is string, otherwise it is sent in a
+   separate packet.
 
 .. method:: HTTPConnection.send(data)
 

Lib/http/client.py

@@ -947,11 +947,11 @@ class HTTPConnection:
     def endheaders(self, message_body=None):
         """Indicate that the last header line has been sent to the server.
 
-        This method sends the request to the server.  The optional
-        message_body argument can be used to pass message body
-        associated with the request.  The message body will be sent in
-        the same packet as the message headers if possible.  The
-        message_body should be a string.
+        This method sends the request to the server.  The optional message_body
+        argument can be used to pass a message body associated with the
+        request.  The message body will be sent in the same packet as the
+        message headers if it is a string, otherwise it is sent as a separate
+        packet.
         """
         if self.__state == _CS_REQ_STARTED:
             self.__state = _CS_REQ_SENT