add javadoc for AbstractView, View, SpatialView

When fixing up a test after a merge commit, I noted that I had to create a View object directly. In the process, I found that many of these classes are public, have public ctors, but have no javadoc. The client API takes AbstractView as an argument, so it may be tempting to just construct one. Is it correct to do so? Maybe. Javadoc doesn't tell me much.

The arguments to the ctors could also be expanded as they're a bit outside code style. Specifically, variable names should be meaningful and at least three or more characters if the variable is to be around for a while.