doc: Enable REPEAT_BRIEF in Doxygen

The detailed documentation of some functions need the brief description
included to make sense.

doc/doxygen/riot.doxyfile

@@ -107,7 +107,7 @@ BRIEF_MEMBER_DESC      = YES
 # brief descriptions will be completely suppressed.
 # The default value is: YES.
 
-REPEAT_BRIEF           = NO
+REPEAT_BRIEF           = YES
 
 # This tag implements a quasi-intelligent brief description abbreviator that is
 # used to form the text in various listings. Each string in this list, if found