message: Quick clarification about the stack nature of the default graph context.
diff: @@ -3558,9 +3558,13 @@ class Graph(object):
  This method should be used if you want to create multiple graphs
  in the same process. For convenience, a global default graph is
  provided, and all ops will be added to this graph if you do not
  - create a new graph explicitly. Use this method with the `with` keyword
  - to specify that ops created within the scope of a block should be
  - added to this graph.
  + create a new graph explicitly.
  +
  + Use this method with the `with` keyword to specify that ops created within
  + the scope of a block should be added to this graph. In this case, once
  + the scope of the `with` is exited, the previous default graph is set again
  + as default. There is a stack, so it's ok to have multiple nested levels
  + of `as_default` calls.
  The default graph is a property of the current thread. If you
  create a new thread, and wish to use the default graph in that
  