[Components][Process] `mustRun()` documentation

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#8655)
| Applies to    | 2.5+
| Fixed tickets | #4187

@weaverryan Sorry, this should be merged into 2.5.
What do you think about being a bit more specific?

>
The `mustRun` method is identical to `run`, except that it will throw a
`ProcessFailedException` if the process couldn't be executed successfully
(i.e. the process exited with a non-zero code)::

As of Symfony 2.5, you can use the :method:`Symfony\\Component\\Process\\Process::mustRun`
method which throws a :class:`Symfony\\Component\\Process\\Exception\\ProcessFailedException`
when the process couldn't be executed successfully::