[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) 
We could use an unordered list for the three ways to use the helper. What do you think?