[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.
I prefer to add an empty line before this and just use `echo` (is there a reason you used `print`?)