A part about PlantUML library documentaion is on my shared directory, the file is "plantuml.pdf", and the interesting point are "1. La librairie PlantUML" and "2. Graphviz".
This part is technical.

Then, for the user documentaion, there are some subtlety for generate a content type diagram, that are explained in the same file at the point "3. L’export graphique"