[gegl] operations: improve gegl:watershed-transform description.

[Jehan <jehanp src gnome org>]

commit 1cf35de73509aaca26a834e75c7035622268514f
Author: Jehan <jehan girinstud io>
Date:   Thu Oct 11 09:22:41 2018 +0200

    operations: improve gegl:watershed-transform description.
    
    It is completely unobvious how to use the operation without looking at
    the code (in particular the fact that it uses the alpha channel to
    detect what is labelled or not, and also the babl formats it will work
    on since you likely don't want your labels converted). Let's add a
    slightly more descriptive text.

 operations/common/watershed-transform.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)
---
diff --git a/operations/common/watershed-transform.c b/operations/common/watershed-transform.c
index 34214f506..990e7433a 100644
--- a/operations/common/watershed-transform.c
+++ b/operations/common/watershed-transform.c
@@ -383,7 +383,11 @@ gegl_op_class_init (GeglOpClass *klass)
     "title",       _("Watershed Transform"),
     "reference-hash", "c5623beeef052a9b47acd178dd420864",
     "categories",  "hidden",
-    "description", _("Labels propagation by watershed transformation"),
+    "description", _("Labels propagation by watershed transformation. "
+                     "Output and expected input are \"YA u32\" grayscale buffers. "
+                     "The Y value represents a label. When a pixel is not labelled, "
+                     "its alpha value has to be set to 0. "
+                     "The mandatory aux buffer is a \"Y u8\" image representing the priority levels."),
     NULL);
 }