docs: clarify dict structured output schemas

[MukundaKatta]

Fixes #556.

Summary

• clarify that dict[str, AllowedType] type-hint schemas describe arbitrary string-keyed maps
• point users to Pydantic models or explicit JSON Schema properties for fixed named fields
• keep README, codegen instructions, and Sphinx source text aligned

Verification

• git diff --check
• docs-only change; no runtime tests run