Fix JDK8 javadoc issues

Fixed errors when running mvn site on JDK8.
These were caused by the backwards-compatibility breaking changes of javadoc in JDK8.
In general, we have been replacing this:

```html
Summary
<p/>
Additional information.
```

With this:

```html
Summary
<p>
Additional information.</p>
```

Can you add the closing `</p>` to each of the paragraphs created as part of this change?