docs: Replace deprecated Virtual tag

Use the (virtual) symbol annotation.

gio/goutputstream.c

@@ -171,7 +171,7 @@ g_output_stream_init (GOutputStream *stream)
 }
 
 /**
- * g_output_stream_write:
+ * g_output_stream_write: (virtual write_fn)
  * @stream: a #GOutputStream.
  * @buffer: (array length=count) (element-type guint8): the buffer containing the data to write. 
  * @count: the number of bytes to write
@@ -199,8 +199,6 @@ g_output_stream_init (GOutputStream *stream)
  *
  * On error -1 is returned and @error is set accordingly.
  * 
- * Virtual: write_fn
- *
  * Returns: Number of bytes written, or -1 on error
  **/
 gssize
@@ -320,7 +318,7 @@ g_output_stream_write_all (GOutputStream  *stream,
 }
 
 /**
- * g_output_stream_writev:
+ * g_output_stream_writev: (virtual writev_fn)
  * @stream: a #GOutputStream.
  * @vectors: (array length=n_vectors): the buffer containing the #GOutputVectors to write.
  * @n_vectors: the number of vectors to write
@@ -353,8 +351,6 @@ g_output_stream_write_all (GOutputStream  *stream,
  * are exceeded. For example, when writing to a local file on UNIX platforms,
  * the aggregate buffer size must not exceed %G_MAXSSIZE bytes.
  *
- * Virtual: writev_fn
- *
  * Returns: %TRUE on success, %FALSE if there was an error
  *
  * Since: 2.60