add unsorted bin to doc

panda/src/doc/howto.control_render_order

@@ -111,10 +111,11 @@ GeomBinFixed.
 The order of all the predefined bins (and their predefined sort
 orders) is as follows:
 
-  10 - 'background' : GeomBinFixed
-  20 - 'default'    : GeomBinNormal (opaque sub-bin)
-  30 - 'default'    : GeomBinNormal (transparent sub-bin)
-  40 - 'fixed'      : GeomBinFixed
+  10 - 'background'  : GeomBinFixed
+  20 - 'opaque'      : GeomBinNormal (opaque sub-bin)
+  30 - 'transparent' : GeomBinNormal (transparent sub-bin)
+  40 - 'fixed'       : GeomBinFixed
+  50 - 'unsorted'    : GeomBinUnsorted
 
 Thus, the 'fixed' bin can be used for things that must be rendered
 correctly relative to each other, but should render after all other