docs: clarify tool-agent input context

[adityasingh2400]

Summary

• Clarify that Agent.as_tool() nested runs receive the tool-call input, not the outer agent transcript automatically.
• Point users who need richer specialist context to explicit tool arguments, input_builder, or direct Runner.run inside a custom function tool.

Test plan

• uv run mkdocs build
• git diff --check

Related to #858.

[seratch]

Thanks for the suggestion. However, adding this sentence there is not relevant and it is actually incorrect.