closes #370

Adds comments to label, legend and caption elements to indicate that if they are hidden, then they will not return a name for their associated elements.

Note that right now this means that 
```
<label for=d hidden>foo</label>
<input id=d title=bar>
```
has _no name_.  The label is still referenced, but because it is hidden it returns an empty name in Safari and Chromium browsers.  Firefox correctly returns "bar" as the name of the input.