Docs: make the meaning of "Error" clearer in the Generics docs #161157

Documentation

I think the Generics docs is confusing in the way it describes errors. Currently, the following code comments are used:

• # INVALID
• # illegal
• # Error
• # Rejected
• # NOT valid
• # Not OK
• # Type error
• # Type checker error

These don't distinguish between actual runtime errors vs type checker warnings in code examples. Only the last one is fully clear.

What do we think?