[gtk+] Add a long description for GdkTexture

[Matthias Clasen <matthiasc src gnome org>]

commit d5afb9e8050a268a6680e066c2fa87db37d82509
Author: Matthias Clasen <mclasen redhat com>
Date:   Mon Dec 4 11:04:02 2017 -0800

    Add a long description for GdkTexture

 gdk/gdktexture.c |   16 ++++++++++++++++
 1 files changed, 16 insertions(+), 0 deletions(-)
---
diff --git a/gdk/gdktexture.c b/gdk/gdktexture.c
index a7250d1..6921297 100644
--- a/gdk/gdktexture.c
+++ b/gdk/gdktexture.c
@@ -40,6 +40,22 @@
 #include "gdkcairo.h"
 
 /**
+ * SECTION:gdktexture
+ * @Short_description: Image data for display
+ * @Title: GdkTexture
+ *
+ * A GdkTexture represents image data that can be displayed on screen.
+ *
+ * There are various ways to create GdkTexture objects from a #GdkPixbuf
+ * or a cairo surface, or other pixel data.
+ *
+ * An important aspect of GdkTextures is that they are immutable - once
+ * the image data has been wrapped in a GdkTexture, it may be uploaded
+ * to the GPU or used in other ways that make it impractical to allow
+ * modification.
+ */
+
+/**
  * GdkTexture:
  *
  * The `GdkTexture` structure contains only private data.