[Console] Add Process Helper documentation
feature #3930 [Console] Add Process Helper documentation (romainneutron)

This PR was merged into the master branch.

Discussion
----------

[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)

Commits
-------

a9d6a8b [Console] Add Process Helper documentation