Guide: Clarify that assigning/comparing different tuple types doesn't work

Currently, the Guide says tuples "are only equivalent if the arity, types, and values are all identical", before presenting an example that uses `==` to compare two tuples whose arity and contained types match. This is misleading, because it implies that `==` can dynamically check whether two tuples have the same arity and contained types, whereas trying to do this would lead to a compiler error.

I tried to avoid destroying the flow of this section, but I'm not sure if I've been successful.
I know you didn't edit this, but wouldn't it be better in the guide to have `assert!(x != y)` instead of making the user guess?