Add more comprehensive documentation for arrow function types

Hi there!

In going over some flowtype annotations we added to our codebase during code review, it came up that:

```
const preventDefault: Event => void = e => e.preventDefault()
```
is seemingly a valid usage of flow. However, all examples outside of the docs have:

```
const preventDefault: (e: Event) => void = e => e.preventDefault()
```

and the docs themselves only point to a model of `(P1: T1): U`

I'd like to propose adding a note in the docs that for arrow function types, parameter name and parenthesis are optional for unary functions (unless this is considered bad practice, then perhaps a note saying that while optional, you should always prefer parens and parameter names in the type sigs)