docs/GL3: clarify core vs compatibility extension support

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

docs/GL3.txt

@@ -7,6 +7,14 @@ infrastructure is complete but it may be the case that few (if any) drivers
 implement the features.
 
 
+OpenGL Core and Compatibility context support
+
+OpenGL 3.1 and later versions are only supported with the Core profile.
+There are no plans to support GL_ARB_compatibility. The last supported OpenGL
+version with all deprecated features is 3.0. Some of the later GL features
+are exposed in the 3.0 context as extensions.
+
+
 Feature                                               Status
 ----------------------------------------------------- ------------------------