[Console] Add Process Helper documentation

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes (symfony/symfony#10627)
| Applies to    | 2.5+
| Fixed tickets | n/a

This PR replaces #3756 (merged prematurely) 
let's not repeat this in all examples and simply use `// ...` to imply that something is missing, people will already know that `$helper` refers to `$this->getHelper('process')`