docs: clarify supported zod schema features

[MukundaKatta]

Summary

• document which common Zod schema shapes work with the strict Structured Outputs subset
• clarify that .optional() should be modeled as nullable for structured outputs
• note that Zod runtime transforms/refinements and TypeScript comments/arbitrary metadata are not sent to the API
• point users to .describe() / helper description for JSON Schema descriptions

Fixes #1017.

Validation

• git diff --check