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.