[gimp] libgimp: add a doc comment to gimp_drawable_get_sub_thumbnail_data().

[Jehan <jehanp src gnome org>]

commit 650d4b57ed4bcac363537ebc4b830edc8a685d60
Author: Jehan <jehan girinstud io>
Date:   Mon Sep 2 11:47:01 2019 +0200

    libgimp: add a doc comment to gimp_drawable_get_sub_thumbnail_data().
    
    This was added in commit e24375e64a together with "Since:" annotation
    updates, which got reverted. Add back just the documentation comment.

 libgimp/gimpdrawable.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)
---
diff --git a/libgimp/gimpdrawable.c b/libgimp/gimpdrawable.c
index ae1dbb7283..ce7bea3648 100644
--- a/libgimp/gimpdrawable.c
+++ b/libgimp/gimpdrawable.c
@@ -133,6 +133,23 @@ gimp_drawable_get_thumbnail (GimpDrawable           *drawable,
   return NULL;
 }
 
+/**
+ * gimp_drawable_get_sub_thumbnail_data:
+ * @drawable:             the drawable ID
+ * @src_x:                the x coordinate of the area
+ * @src_y:                the y coordinate of the area
+ * @src_width:            the width of the area
+ * @src_height:           the height of the area
+ * @dest_width: (inout):  the requested thumbnail width  (<= 1024 pixels)
+ * @dest_height: (inout): the requested thumbnail height (<= 1024 pixels)
+ * @bpp: (out):           the bytes per pixel of the returned thumbnail data
+ *
+ * Retrieves thumbnail data for the drawable identified by @drawable.
+ * The thumbnail will be not larger than the requested size.
+ *
+ * Returns: (transfer full) (nullable): thumbnail data or %NULL if
+ *          @drawable is invalid.
+ **/
 guchar *
 gimp_drawable_get_sub_thumbnail_data (GimpDrawable *drawable,
                                       gint          src_x,