A new version of the Plant SDK documentation that covers the 2017 through 2022 releases of AutoCAD Plant 3D is available for download in the AutoCAD P&ID and Plant 3D Developer Center: https://www.autodesk.com/developer-network/platform-technologies/autocad-p-id-and-plant-3d
A link to download the latest documentation package can be found under both the Learning and Tools sections.