[Suggestion] Update documentation for the session interface to clarify how it internally works

🔴 Required Information

Is your feature request related to a specific problem?

While implementing a custom session.Session, it was confusing that my events weren't taken into account by the agent.

ie

Prompt : "Reply to general questions"

If my GetResponse is

	return &session.CreateResponse{
		Session: sessionImplementation,
	}, nil

The response to "What is the capital city of Germany" will be "Hello, I am ready to help!"

While what is expected is "The capital city of Germany is Berlin." which happens only when a reference is passed :

	return &session.CreateResponse{
		Session: &sessionImplementation,
	}, nil

This happens because :

1. Session is saved at https://github.com/google/adk-go/blob/03705a53d99ea9b2e794ef5328f8f70cb2c90e43/runner/runner.go#L130
2. Events reference is updated at https://github.com/google/adk-go/blob/main/runner/runner.go#L172

Proposed Solution

• Update documentation in https://github.com/google/adk-go/blob/v0.4.0/session/service.go#L72-L75 indicating that a reference to the session object is expected.

• (Potentially), do the same in AppendEvent indicating that the session argument should be modified directly through the request.

I

Impact on your work

This is confusing when trying to implement a custom session, as it often means that the only way to know how it works internally is to check the source code directly/ the inmemory implementation.