Update chain docs

[vshulman]

Clarifying LLM section of chains guide

[marius-baseten]

Thanks a lot for contributing to making the docs better!
Overall this is all fine, just one discussion point for this section here:

I assume your intent for the change here was to make it less "technical bulky" and more accessible. But in this case it waters down the message that it really is a type error - unless you add the protocol annotation.

If someone has type checking set-up in their CI, they might be surprised by getting a type error (or think that our example code is messy). So I think writing "can be substituted" is a bit to lenient. And please keep the ` backticks around identifiers from code examples.