Add language about implicit flush and command completion

Copied language from the glXSwapBuffers manual page about the implicit
glFlush and expected command completion.  This just codifies what
people already expect from glXCopySubBufferMESA.  The intention of
this command is to work like glXSwapBuffers but on a sub-rectangle of
the drawable.

Acked-by: Brian Paul <brianp@vmware.com>

docs/MESA_copy_sub_buffer.spec

@@ -16,7 +16,7 @@ Status
 
 Version
 
-    Last Modified Date:  8 June 2000
+    Last Modified Date:  12 January 2009
 
 Number
 
@@ -69,6 +69,12 @@ Additions to Chapter 3 of the GLX 1.3 Specification (Functions and Errors)
     <width> and <height> indicate the size in pixels.  Coordinate (0,0)
     corresponds to the lower-left pixel of the window, like glReadPixels.
 
+    If dpy and drawable are the display and drawable for the calling
+    thread's current context, glXCopySubBufferMESA performs an
+    implicit glFlush before it returns.  Subsequent OpenGL commands
+    may be issued immediately after calling glXCopySubBufferMESA, but
+    are not executed until the copy is completed. 
+
 GLX Protocol
 
     None at this time.  The extension is implemented in terms of ordinary
@@ -84,5 +90,7 @@ New State
 
 Revision History
 
-    8 June 2000 - initial specification
+    12 January 2009 Ian Romanick - Added language about implicit flush
+                                   and command completion.
+    8 June 2000     Brian Paul   - initial specification