gh-111112: Avoid potential confusion in TCP server example. (#111113)

Improve misleading TCP server docs and example.

socket.recv(), as documented by the Python reference documentation,
returns at most `bufsize` bytes, and the underlying TCP protocol means
there is no guaranteed correspondence between what is sent by the client
and what is received by the server.

This conflation could mislead readers into thinking that TCP is
datagram-based or has similar semantics, which will likely appear to
work for simple cases, but introduce difficult to reproduce bugs.

Author: aidanholm

Doc/library/socketserver.rst

@@ -494,7 +494,7 @@ This is the server side::
        def handle(self):
            # self.request is the TCP socket connected to the client
            self.data = self.request.recv(1024).strip()
-           print("{} wrote:".format(self.client_address[0]))
+           print("Received from {}:".format(self.client_address[0]))
            print(self.data)
            # just send back the same data, but upper-cased
            self.request.sendall(self.data.upper())
@@ -525,8 +525,9 @@ objects that simplify communication by providing the standard file interface)::
 
 The difference is that the ``readline()`` call in the second handler will call
 ``recv()`` multiple times until it encounters a newline character, while the
-single ``recv()`` call in the first handler will just return what has been sent
-from the client in one ``sendall()`` call.
+single ``recv()`` call in the first handler will just return what has been
+received so far from the client's ``sendall()`` call (typically all of it, but
+this is not guaranteed by the TCP protocol).
 
 
 This is the client side::