Update SerialBase sync() function description

Author: pennam

drivers/include/drivers/SerialBase.h

@@ -119,7 +119,9 @@ class SerialBase : private NonCopyable<SerialBase> {
      */
     void send_break();
 
-    /** Flush hardware buffers associated with serial port
+    /** Ensures all data stored in peripheral buffers is transmitted to the bus
+     * 
+     *  The function shall block until all the content of peripheral buffers is transmitted.
      *
      *  @return         0 on success, negative error code on failure
      */