Remove out-of-date explanation in doc strings,

as reported by Eric C. Newton.

Lib/compiler/visitor.py

@@ -26,9 +26,7 @@ class ASTVisitor:
     the order during a postorder walk, because it is called _after_
     the walk has occurred.)  The ASTVisitor modifies the visitor
     argument by adding a visit method to the visitor; this method can
-    be used to visit a particular child node.  If the visitor method
-    returns a true value, the ASTVisitor will not traverse the child
-    nodes.
+    be used to visit a particular child node.
 
     XXX The interface for controlling the preorder walk needs to be
     re-considered.  The current interface is convenient for visitors