write use-ed validators to output_schema root node

[zsimjee]

This makes it possible to use the .use syntax with guard.call()

[zsimjee]

Alright making a couple of changes here

1. Update docstrings to say that call on guards constructed using use and use_many is only supported for string-style guards
2. Throw an exception when use is called on pydantic-style or object-style guards

[zsimjee]

Ready for review

[ShreyaR]

lgtm