Add negative number clarification.

To be a bit more specific, unary operators are not given higher operator precedence. For example, this fails:

```haskell
ghci> 2 + -3
```

Lack of parentheses / higher operator precedence creates ambiguity in this example as well:

```haskell
ghci> f - 3
```

Is it `f (-3)` or ``f `-` 3``?

On a side note, my editor removes trailing whitespace. If this is an issue please let me know or cherry pick b754d8b. If you're worried about clobbering `git blame` history, there's always the ignore whitespace option (`-w`).