doc: document auto_init

sys/include/auto_init.h

@@ -36,6 +36,13 @@
 extern "C" {
 #endif
 
+/**
+ * @brief Initializes all high level modules that do not require parameters for
+ *        initialization or uses default values.
+ *
+ *        This function gets called - if not explicitely disabled - by @ref
+ *        kernel_init right before jumping into @e main.
+ */
 void auto_init(void);
 
 #ifdef __cplusplus