Handle uses in docs

[samskivert]

Javadoc supports referencing other types from within the documentation. We want those references to be live in the rendered documentation. Probably that means something like turning:

Calls {@link Foo#fizBang} with the {@link Frobble}.

into:

<def ...>
  <doc>Calls fizBang with the Frobble.
    <use name="fizBang" id="...Foo.fizBang" start="6"/>
    <use name="Frobble" id="...Frobble" start="24"/>
  </doc>
</def>