The digiKam documentation comes as a separate package usually called digikam-doc) which you have to install for local use. But you can read it online as well (some options might differ from your local installation because the online manuals belongs to the development versions):
A wiki page hosted by KDE project is under contruction. Every one can contribute to add and fix wiki contents. Look here for details.
API and Database schema
If you plan developing for digiKam you might find these links interesting:
Online Source Code
We'll be happy to take up your remarks, corrections or patches to make our manuals even better, we care for details like wrong plural forms etc.
If you'd like to contribute to the documentation, have a look at the contrib page.