xtimer/xtimer.c document mutex_thread_t

comment for mutex_thread_t

sys/xtimer/xtimer.c

@@ -38,6 +38,10 @@
 #define ENABLE_DEBUG 0
 #include "debug.h"
 
+/*
+ * @brief: struct for mutex lock with timeout
+ * xtimer_mutex_lock_timeout() uses it to give information to the timer callback function
+ */
 typedef struct {
     mutex_t *mutex;
     thread_t *thread;