Recognize doc comments on AssignmentExpressions

Modifies doc_comment.js to recognize doc comments on AssignmentExpression nodes. This means that the following doc comments will be recognized:

```javascript
var o = {};

// The string "foo".
o.foo = "foo";

/** @returns {String} */
o.prop3 = function() { return something(); };

// docs for f2 (node.js example)
module.exports.f2 = function() { return 7; };
```

Recognizing doc comments on **all** AssignmentExpressions is quite wasteful and will include a lot of comments that aren't intended to be doc comments. However, at least for my purposes, good coverage (no false negatives) is more important than avoiding false positives. It is possible that non-doc comments could clobber the real doc comments on nodes, but I have not seen that to be an issue in my own usage so far.