bpo-3530: advice fix_missing_locations usage on new node introduction with NodeTransformer

Author: isidentical

Doc/library/ast.rst

@@ -300,7 +300,7 @@ and classes for traversing abstract syntax trees:
       class RewriteName(NodeTransformer):
 
           def visit_Name(self, node):
-              return copy_location(Subscript(
+              return Subscript(
                   value=Name(id='data', ctx=Load()),
                   slice=Index(value=Constant(value=node.id)),
                   ctx=node.ctx
@@ -314,6 +314,11 @@ and classes for traversing abstract syntax trees:
    statement nodes), the visitor may also return a list of nodes rather than
    just a single node.
 
+   If :class:`NodeTransformer` introduces new nodes (that weren't part of
+   original tree) without giving them location information (such as
+   :attr:`lineno`), :func:`fix_missing_locations` should be called with
+   new tree to apply location information for compiler.
+
    Usually you use the transformer like this::
 
       node = YourTransformer().visit(node)

Misc/NEWS.d/next/Documentation/2019-11-17-11-53-10.bpo-3530.8zFUMc.rst

@@ -0,0 +1,2 @@
+In ast doc, fix misleading NodeTransformer example and add
+fix_missing_locations advice.