In fact the Modeling and Implementation Guide it is an official documentation.
Oracle® Fusion Middleware Modeling and Implementation Guide for Oracle Business Process Management 11g Release 1 (126.96.36.199)
31 Working with Case Management
But I would also strongly recommend you to read this series of posts from the Oracle A-Team (start from the end of the list below):
Case Management In-Depth: Cases & Case Activities Part 2 – Case Rules
Case Management In-Depth: Cases & Case Activities Part 1 – Acivity Scope
Case Management In-Depth: Stakeholders & Permissions
Case Management Part 3: Runtime Lifecycle of a Project
Case Management Part 2: Anatomy of a Project
Case Management Part 1: An Introduction
but I know all these documents. I'm asking about "developers documentation" only, which should be but does not exist.