explain generalization when talking about type inference

[lcnr]

generalization is a core - and subtle - part of type inference and currently not explained in the guide. We should change that. We should talk about equality in general, the current section only mentions how to use equality, but doesn't actually explain how it works https://rustc-dev-guide.rust-lang.org/type-inference.html?highlight=equate#enforcing-equality--subtyping