Add definition of purpose and use of networking module

The purpose of the networking module can sometimes be misunderstood. This adds
a definition and explanation of what the networking module is and what it can be
used for.

include/mbedtls/net_sockets.h

@@ -1,7 +1,22 @@
 /**
  * \file net_sockets.h
  *
- * \brief Network communication functions
+ * \brief   Network sockets abstraction layer to integrate Mbed TLS into a
+ *          BSD-style sockets API.
+ *
+ *          The network sockets module provides an example integration of the
+ *          Mbed TLS library into a BSD sockets implementation. The module is
+ *          intended to be both an example of how Mbed TLS can be integrated
+ *          into a networking stack, and also act as Mbed TLS's integration on
+ *          the supported platforms.
+ *
+ *          The module is intended only for the use of the Mbed TLS library and
+ *          is not intended to be used by third party application software.
+ *
+ *          The supported platforms are as follows:
+ *              * Microsoft Windows and Windows CE
+ *              * POSIX/Unix platforms including Linux, OS X
+ *
  */
 /*
  *  Copyright (C) 2006-2015, ARM Limited, All Rights Reserved