DOXYGEN: Review of high prio headers from engines

This is a doxygen review for 4 header files that have been
identified as high priority for GSoD:

- advancedDetector.h
- metaengine.h
- engine.h
- util.h

It also adds doxy groups to other headers from this folder to
make them appear properly in the structure.

doc/doxygen/groups.dox

@@ -1,38 +1,37 @@
 /**
 
 @defgroup common Common API
-@brief Common functions and structures.
+@brief Common classes, functions, and structures.
 
 */
 
 /**
-
-@defgroup lua Lua API
+@defgroup lua Lua
 @ingroup common
 @brief Functions for the Lua programming language. See @linkLua.
-
 */
 
 /**
 
-@defgroup graphics Graphics API
-@brief API related to on-screen graphics - surfaces, pixels, cursors, fonts, palettes, and renderers.
+@defgroup audio Audio API
+@brief APIs for managing in-game audio.
 
 */
 
 /**
 
-@defgroup image Image API
-@brief API of various image decoders used in engines.
+@defgroup engines Engine API
+@brief API related to engines.
 
 */
 
 /**
 
-@defgroup audio Audio API
-@brief APIs for managing in-game audio.
+@defgroup graphics Graphics API
+@brief API related to on-screen graphics - surfaces, pixels, cursors, fonts, palettes, and renderers.
 
 */
+
 /**
 
 @defgroup image Image API