Recognize doc comments on AssignmentExpressions with RHS of type FunctionExpression

Modifies doc_comment.js to recognize doc comments on AssignmentExpression nodes whose RHS is a FunctionExpression. This means that the following doc comments will be recognized:

```javascript
var o = {};

// Returns foo.
o.foo = function() { return "foo"; };

/** @returns {String} */
o.prop3 = function() { return something(); };

// docs for f2 (node.js example)
module.exports.f2 = function() { return 7; };
```