update/extend doxygen documentation

It's a pity that even with this patch we still are fare away from having
the whole API documented.  However, at least we have a more solid
foundation.  Updates not only extend the documentation, but also make
sure it is rendered properly in the doxygen HTML.

Change-Id: I1344bd1a6869fb00de7c1899a8db93bba9bafce3
diff --git a/include/osmocom/core/msgb.h b/include/osmocom/core/msgb.h
index afb887c..f5dd323 100644
--- a/include/osmocom/core/msgb.h
+++ b/include/osmocom/core/msgb.h
@@ -29,13 +29,7 @@
  *  @{
  */
 
-/*! \file msgb.h
- *  \brief Osmocom message buffers
- * The Osmocom message buffers are modelled after the 'struct skb'
- * inside the Linux kernel network stack.  As they exist in userspace,
- * they are much simplified.  However, terminology such as headroom,
- * tailroom, push/pull/put etc. remains the same.
- */
+/*! \file msgb.h */
 
 #define MSGB_DEBUG