Documentation for BfsNodeVisitor is inaccurate

[DavidDTA]

There are two inaccuracies, both of which are demonstrated by this Ellie

• The documentation says

  it holds always that length path == distance - 1

  but actually length path == distance + 1

• The documentation says

  the path of contexts from the root to the current node is passed instead of just the current node's context

  but the path is actually passed in the reverse order, from the current node to the root