document how to switch to heuristic builder

fixes BZDNG-375

modules/conop/doc/connectivity.rst

@@ -258,3 +258,15 @@ The RuleBasedBuilder class
      
      :rtype: list of :class:`~ost.mol.AtomHandle` instances
 
+Changing the default builder
+---------------------------------------------------------------------------------
+
+The default builder can be specified with :func:`SetDefaultBuilder`. Before being
+able to set a builder, it needs to be registered with :func:`RegisterBuilder`.
+By default, there is always a builder called "HEURISTIC" registered. If, for some
+reason your are currently using the :class:`RuleBasedBuilder` and you would like
+to switch to that builder, call
+
+.. code-block:: python
+
+  conop.SetDefaultBuilder("HEURISTIC")

modules/conop/pymod/__init__.py

@@ -64,7 +64,12 @@ def RegisterBuilder(builder, name):
   
 def SetDefaultBuilder(builder_name):
   '''
-  Set the builder with the given name as the default.
+  Set the builder with the given name as the default. You will have to register
+  a builder with :func:`RegisterBuilder` before you will be able to set it as
+  the default.
+
+  :raises: :exc:`RuntimeError` when trying to set a builder as the default that
+    has not been registered yet.
   '''
   conop_inst=Conopology.Instance()
   conop_inst.SetDefaultBuilder(builder_name)