[Components][Process] `mustRun()` documentation
feature #4201 [Components][Process] `mustRun()` documentation (xabbuh)

This PR was submitted for the master branch but it was merged into the 2.5 branch instead (closes #4201).

Discussion
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[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.

Commits
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ad6a340 [Components][Process] `mustRun()` documentation