The basic documentation is built-in -- in VBA use the Object Browser and
Help. If you put your cursor in any keyword -- in code or in the Immediate
window -- and press F1, you get the documentation for that keyword. Apart
from that, there's the entire MSDN library on the Microsoft site.
There are two parts to the documentation you need --
1. VBA as a language. This is the same for any application (that is, Word
VBA is the same language as Excel VBA or CorelDraw VBA, etc).
2. The object models for the application you're dealing with. In this case,
Word and to a lesser extent Office (which handles the menus); plus any other
libraries you choose to add to your project.