[gnome-photos] camera-cache: Add a comment

[Debarshi Ray <debarshir src gnome org>]

commit e704d1e7e5e9d4af9b06f77f31700acb64c8b30d
Author: Debarshi Ray <debarshir gnome org>
Date:   Mon Jan 2 17:50:12 2017 +0100

    camera-cache: Add a comment

 src/photos-camera-cache.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)
---
diff --git a/src/photos-camera-cache.c b/src/photos-camera-cache.c
index a92d82b..2edb529 100644
--- a/src/photos-camera-cache.c
+++ b/src/photos-camera-cache.c
@@ -68,6 +68,11 @@ photos_camera_cache_cursor_next (GObject *source_object, GAsyncResult *res, gpoi
       goto out;
     }
 
+  /* Note that the following SPARQL query:
+   *   SELECT nfo:manufacturer (<(foo)>) nfo:model (<(foo)>) WHERE {}
+   * ... will not return an empty cursor, but:
+   *   (null), (null)
+   */
   if (!success)
     {
       g_task_return_new_error (task, PHOTOS_ERROR, 0, "Cursor is empty — possibly wrong SPARQL query");