[gegl] gegl-xml: Add a comment for clarification

[Debarshi Ray <debarshir src gnome org>]

commit 30de126519139bd93d5edd4bd03423434ee49b3c
Author: Debarshi Ray <debarshir gnome org>
Date:   Mon Jan 18 14:17:53 2016 +0100

    gegl-xml: Add a comment for clarification
    
    https://bugzilla.gnome.org/show_bug.cgi?id=760784

 gegl/gegl-xml.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/gegl/gegl-xml.c b/gegl/gegl-xml.c
index 30dc549..0ccf47f 100644
--- a/gegl/gegl-xml.c
+++ b/gegl/gegl-xml.c
@@ -1129,6 +1129,9 @@ add_stack (SerializeState *ss,
               {
                 GeglNode *graph       = g_object_get_data (G_OBJECT (source_node),
                                                            "graph");
+                /* if source_node is a proxy then make it point to the
+                 * actual node
+                 */
                 if (graph)
                   source_node = graph;
                 iter = source_node;