Box2 and Box3 - Docs and parameter tweak

I updated the Box3 docs. I also added a an `optionalTarget` parameter to `Box2.getParameter()` and `Box3.getParameter()`. Every other method had an optional target for any method that called a `new Three.Object()`, but getParameter seemed to miss it.

The changes passed the unit tests but I didn't incorporate into the build. If the parameter tweak needs to be taken out, let me know.