We should document how to combine search tools with other tools

**Is your feature request related to a problem? Please describe.**
If you try to use a VertexAISearchTool with other tools, you get an error that search tools cannot be combined with other tools: "Multiple tools are supported only when they are all search tools."

The documentation doesn't describe this, even in Implementation Considerations:
https://google.github.io/adk-docs/grounding/vertex_ai_search_grounding/#implementation-considerations

**Describe the solution you'd like**

We should include this solution that I found in the Google ADK Chat and explain what an AgentTool does to solve this problem: https://chat.google.com/room/AAAAYDpR2kM/TWgBgWeNQbU/wn5KoHWuI8U?cls=10