Improve docs about /* @flow */ and cross-link in User Guide

I have some code with running flow server. `flow` gives as response `No errors!` when i add the following code to the file:

``` javascript
// ... some code
/* @flow */
function add(num1, num2) {
return num1 + num2;
}
var x = add(3, '0');
console.log(x);
// ... some code
```

Running `flow` still gives back `No errors!`. Running `flow check` gives back `Found 0 errors`
running `flow check --all` does echo the errors (among others ofcourse). Is this normal behavior (I started experimenting with flowtype)?