clarifying iterator trait documentation

I found these things to be ambiguous, or at least worth stating explicitly to reduce the amount a user/developer needs to think about the API.
There are a few places outside my changes that already refer to functions without the parens, e.g. 

> The `Iterator` trait has one
> unimplemented method, `next`. All other methods are derived through default
> methods to perform operations such as `zip`, `chain`, `enumerate`, and `fold`.

Are there any guidelines for when parens should or should not be included in docs? Should I change the others to use parens?