[socket.acceptor.cons] Add postcondition for native_handle()

Fixes NB GB-17 (PDTS)

Author: jwakely

src/sockets.tex

@@ -3611,6 +3611,8 @@
 \item
 \tcode{enable_connection_aborted()} returns the same value as \tcode{rhs.enable_connection_aborted()} prior to the constructor invocation.
 \item
+\tcode{native_handle()} returns the same value as \tcode{rhs.native_handle()} prior to the constructor invocation.
+\item
 \tcode{protocol_} is equal to the prior value of \tcode{rhs.protocol_}.
 \item
 \tcode{rhs.is_open() == false}.