doxygen: document all code

This patch update doxygen configuration to have all possible functions
documented (even the one without doxygen formated comments). It can be
really useful to have that in case we are trying to get some information
on call graph for example.

doxygen.cfg

@@ -351,7 +351,7 @@ LOOKUP_CACHE_SIZE      = 0
 # Private class members and static file members will be hidden unless
 # the EXTRACT_PRIVATE respectively EXTRACT_STATIC tags are set to YES
 
-EXTRACT_ALL            = NO
+EXTRACT_ALL            = YES
 
 # If the EXTRACT_PRIVATE tag is set to YES all private members of a class
 # will be included in the documentation.