ecore_audio: add @brief doc for struct Ecore_Audio_Vio.

It's simply a copy of the one given in the following typedef:
  typedef struct _Ecore_Audio_Vio Ecore_Audio_Vio;

src/lib/ecore_audio/Ecore_Audio.h

@@ -82,6 +82,7 @@ typedef struct _Ecore_Audio_Module Ecore_Audio_Module;
 typedef struct _Ecore_Audio_Object Ecore_Audio_Object;  /**< The audio object */
 
 /*
+ * @brief Structure to hold the callbacks needed to implement virtual file IO
  * @since 1.8
  */
 struct _Ecore_Audio_Vio {