doc: fix quat_math documentation

sys/quad_math/quad.h

@@ -82,15 +82,15 @@ typedef unsigned long long u_quad_t;
 #define QUAD_MAX (LLONG_MAX)
 #define UQUAD_MAX (ULLONG_MAX)
 
-/*
+/**
  * Depending on the desired operation, we view a `long long' (aka quad_t) in
  * one or more of the following formats.
  */
 union uu {
-    quad_t  q;      /* as a (signed) quad */
-    u_quad_t uq;    /* as an unsigned quad */
-    int sl[2];      /* as two signed ints */
-    u_int   ul[2];  /* as two unsigned ints */
+    quad_t  q;      /**< as a (signed) quad */
+    u_quad_t uq;    /**< as an unsigned quad */
+    int sl[2];      /**< as two signed ints */
+    u_int   ul[2];  /**< as two unsigned ints */
 };
 
 /*