docs: Improve description of gdk_gl_context_is_legacy()

Explain why this function is available, and why you may need it.

https://bugzilla.gnome.org/show_bug.cgi?id=756142

diff --git a/gdk/gdkglcontext.c b/gdk/gdkglcontext.c
index d7420e9b3f183b1e5f52c88bc8ce15f4ff83721f..06cd01922cdcf33169a38f50b1d650cc9be1b377 100644 (file)
--- a/gdk/gdkglcontext.c
+++ b/gdk/gdkglcontext.c
@@ -564,7 +564,20 @@ gdk_gl_context_get_required_version (GdkGLContext *context,
  *
  * Whether the #GdkGLContext is in legacy mode or not.
  *
- * The #GdkGLContext must be realized.
+ * The #GdkGLContext must be realized before calling this function.
+ *
+ * When realizing a GL context, GDK will try to use the OpenGL 3.2 core
+ * profile; this profile removes all the OpenGL API that was deprecated
+ * prior to the 3.2 version of the specification. If the realization is
+ * successful, this function will return %FALSE.
+ *
+ * If the underlying OpenGL implementation does not support core profiles,
+ * GDK will fall back to a pre-3.2 compatibility profile, and this function
+ * will return %TRUE.
+ *
+ * You can use the value returned by this function to decide which kind
+ * of OpenGL API to use, or whether to do extension discovery, or what
+ * kind of shader programs to load.
  *
  * Returns: %TRUE if the GL context is in legacy mode
  *