[gegl] sobel: Add comment

[Mukund Sivaraman <muks src gnome org>]

commit 1c86b3291910859385af6879cdb489abb1be7937
Author: Mukund Sivaraman <muks banu com>
Date:   Wed Oct 8 17:58:55 2014 +0530

    sobel: Add comment

 operations/common/edge-sobel.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/operations/common/edge-sobel.c b/operations/common/edge-sobel.c
index 97b5061..6628860 100644
--- a/operations/common/edge-sobel.c
+++ b/operations/common/edge-sobel.c
@@ -249,6 +249,8 @@ edge_sobel (GeglBuffer          *src,
       ptr += src_width * 4;
     }
 
+  /* Fill in the 1px top and bottom borders of the source buffer from
+     its neighbour's pixel. */
   ptr = src_buf;
   ptr2 = src_buf + (src_rect->height + 1) * src_width * 4;
   for (x = 0; x < src_width; x++)