[gegl] operations: update "gegl:value-propagate" description.

[Jehan Pagès <jehanp src gnome org>]

commit f888b731c2e60cecc8f398fa7b9c1c7cacad105b
Author: Jehan <jehan girinstud io>
Date:   Wed Mar 21 19:13:44 2018 +0100

    operations: update "gegl:value-propagate" description.
    
    The most 2 famous filters based on this operation are "erode" and
    "dilate" which are meant to work for the white and black colors.
    "gegl:value-propagate" is the generic version, which works for any
    color, also for alpha channel and such.
    The update is mostly to add the terms "erode" and "dilate" into the
    description somewhere so that this operation will be raised in the
    action search in GIMP along with the basic erode/dilate filters. This
    way, people will understand they can go further with it.

 operations/common-gpl3+/value-propagate.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/operations/common-gpl3+/value-propagate.c b/operations/common-gpl3+/value-propagate.c
index 836d9e0..5b62961 100644
--- a/operations/common-gpl3+/value-propagate.c
+++ b/operations/common-gpl3+/value-propagate.c
@@ -642,7 +642,7 @@ gegl_op_class_init (GeglOpClass *klass)
     "categories",  "distort",
     "reference-hash", "0bce414c416e2d705d0b9ecd8333d93b",
     "license",     "GPL3+",
-    "description", _("Propagate certain colors to neighboring pixels."),
+    "description", _("Propagate certain values to neighboring pixels. Erode and dilate any color or 
opacity."),
     NULL);
 }