Documentation improvement

Document the size requirements on output buffer in g_base64_encode_close.
Patch by Christian Persch, bug 609564.

glib/gbase64.c

@@ -185,6 +185,10 @@ g_base64_encode_step (const guchar *in,
  *
  * Flush the status from a sequence of calls to g_base64_encode_step().
  *
+ * The output buffer must be large enough to fit all the data that will
+ * be written to it. It will need up to 4 bytes, or up to 5 bytes if
+ * line-breaking is enabled.
+ *
  * Return value: The number of bytes of output that was written
  *
  * Since: 2.12