gbase64: Document that g_base64_encode_close() does not nul-terminate

Signed-off-by: Philip Withnall <withnall@endlessm.com>
diff --git a/glib/gbase64.c b/glib/gbase64.c
index a6cbea4..7bca5ab 100644
--- a/glib/gbase64.c
+++ b/glib/gbase64.c
@@ -192,6 +192,8 @@
  * be written to it. It will need up to 4 bytes, or up to 5 bytes if
  * line-breaking is enabled.
  *
+ * The @out array will not be automatically nul-terminated.
+ *
  * Returns: The number of bytes of output that was written
  *
  * Since: 2.12