The WindowBuilder docs that you reference are quite extensive (nearly 300 pages), but they are intended to be docs about how to use WindowBuilder itself. They are intentionally not intended to be documentation for the Swing, SWT or GWT APIs and should not be considered to be substitutes for those. There are plenty of good sources of information on those APIs.
If you are interested in info on Swing, the primary source should be the docs and tutorials provided by Sun/Oracle. Once you have a basic understanding of Swing itself, I think you will find the WindowBuilder docs to be of great help in learning WindowBuilder which itself will be great help to you in rapidly creating Swing UIs.
I should also point out that the WindowBuilder docs in place today are actually better than those in place prior to the Google acquisition of Instantiations because they represent a further six months of work and refinement. While the product has seen a change in legal ownership, it has not actually "changed hands" in the literal sense. The exact same team that developed the tool at Instantiations has continued to work on it at Google and is also forming the core of the committer team at Eclipse.org. This continuity of development stretches eight years into the past and will continue well into the future.