some more Doxygen header text for datagram, osmux and stream module

Change-Id: Ied83e6b117a420d734a88e3aff925a874c3dd520

src/datagram.c

@@ -23,6 +23,11 @@
 
 /*! \addtogroup datagram Osmocom Datagram Socket
  *  @{
+ *
+ *  This code is intended to abstract any use of datagram type sockets,
+ *  such as UDP.  It offers both transmitter and receiver side
+ *  implementations, fully integrated with the libosmocore select loop
+ *  abstraction.
  */
 
 /*! \file datagram.c

src/osmux.c

@@ -25,6 +25,15 @@
 
 /*! \addtogroup osmux Osmocom Multiplex Protocol
  *  @{
+ *
+ *  This code implements a variety of utility functions related to the
+ *  OSMUX user-plane multiplexing protocol, an efficient alternative to
+ *  plain UDP/RTP streams for voice transport in back-haul of cellular
+ *  networks.
+ *
+ *  For information about the OSMUX protocol design, please see the
+ *  OSMUX reference manual at
+ *  http://ftp.osmocom.org/docs/latest/osmux-reference.pdf
  */
 
 /*! \file osmux.c

src/stream.c

@@ -30,6 +30,11 @@
 
 /*! \addtogroup stream Osmocom Stream Socket
  *  @{
+ *
+ *  This code is intended to abstract any use of stream-type sockets,
+ *  such as TCP and SCTP.  It offers both server and client side
+ *  implementations, fully integrated with the libosmocore select loop
+ *  abstraction.
  */
 
 /*! \file stream.c