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/src/crcXXgen.c.tpl b/src/crcXXgen.c.tpl
index 7e45c11..1a69e85 100644
--- a/src/crcXXgen.c.tpl
+++ b/src/crcXXgen.c.tpl
@@ -24,6 +24,7 @@
 
 /*! \addtogroup crcgen
  *  @{
+ *  \brief Osmocom generic CRC routines
  */
 
 /*! \file crcXXgen.c