[doc] Put CRC-16 and generic CRC code in one Doxygen module

.. and add missing API documentation bits

Change-Id: I67119894bcbf8c779426a0272bae4c5ce1fbd1ed
diff --git a/include/osmocom/core/crcXXgen.h.tpl b/include/osmocom/core/crcXXgen.h.tpl
index 2c909f9..823f21f 100644
--- a/include/osmocom/core/crcXXgen.h.tpl
+++ b/include/osmocom/core/crcXXgen.h.tpl
@@ -22,7 +22,7 @@
 
 #pragma once
 
-/*! \addtogroup crcgen
+/*! \addtogroup crc
  *  @{
  * \file crcXXgen.h.tpl */