[gegl] edge-sobel: add doc-description to keep_signal property

[Øyvind Kolås <ok src gnome org>]

commit ffb744b55adee79c8a98507a4e5413a1bd3466cd
Author: Øyvind Kolås <pippin gimp org>
Date:   Tue Oct 7 15:35:23 2014 +0200

    edge-sobel: add doc-description to keep_signal property

 operations/common/edge-sobel.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/operations/common/edge-sobel.c b/operations/common/edge-sobel.c
index 5083f35..9b91147 100644
--- a/operations/common/edge-sobel.c
+++ b/operations/common/edge-sobel.c
@@ -29,6 +29,7 @@ property_boolean (horizontal,  _("Horizontal"), TRUE)
 property_boolean (vertical,  _("Vertical"), TRUE)
 
 property_boolean (keep_signal,  _("Keep Signal"), TRUE)
+     description ("Keep negative values in result, when off the absolute value of the result is used 
instead.")
 
 #else