[Bug #1475554] Strengthen text to say 'must' instead of 'should'

Doc/lib/libsocket.tex

@@ -548,7 +548,7 @@ are described in \ref{bltin-file-objects}, ``File Objects.'')
 The file object references a \cfunction{dup()}ped version of the
 socket file descriptor, so the file object and socket object may be
 closed or garbage-collected independently.
-The socket should be in blocking mode.
+The socket must be in blocking mode.
 \index{I/O control!buffering}The optional \var{mode}
 and \var{bufsize} arguments are interpreted the same way as by the
 built-in \function{file()} function; see ``Built-in Functions''
@@ -647,7 +647,7 @@ Timeout mode internally sets the socket in non-blocking mode.  The
 blocking and timeout modes are shared between file descriptors and
 socket objects that refer to the same network endpoint.  A consequence
 of this is that file objects returned by the \method{makefile()}
-method should only be used when the socket is in blocking mode; in
+method must only be used when the socket is in blocking mode; in
 timeout or non-blocking mode file operations that cannot be completed
 immediately will fail.