Update docs for node-visitor plugins

[hegemonic]

We need to update the docs to explain the changes to node-visitor plugins in 3.3.0:

• exports.astNodeVisitor is the new hotness. It visits Esprima-style/Mozilla Parser API-style nodes.
• exports.nodeVisitor is deprecated. It visits Rhino nodes. It only works if you're using Rhino.

[hegemonic]

Fixed on master.

I deleted the code sample, rather than updating it to use exports.astNodeVisitor, for two reasons:

1. It was incomplete--it called a function that was not defined in the example.
2. It didn't show an example of the source code that it might be able to modify, which made it extremely confusing.
3. Because of issue 2, I wasn't confident that I could update the plugin.