Worked on enhancing developer-facing documentation for the Vertex AI RAG Engine within the google/adk-docs repository. Focused on creating a comprehensive section in built-in-tools.md that details the vertex_ai_rag_retrieval feature, including a Python usage example to guide users in retrieving private data through the agent development kit. Emphasized clarity and discoverability to streamline onboarding and improve developer productivity when integrating Vertex AI RAG with ADK. Utilized Markdown for structured documentation and Python for illustrative code samples. No runtime defects or regressions were introduced, reflecting a careful, documentation-driven approach that prioritized accuracy and usability for the developer community.