Add note about invokable controller services
feature #3966 [Cookbook][Controller] Add note about invokable controller services (kbond)

This PR was merged into the master branch.

Discussion
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[Cookbook][Controller] Add note about invokable controller services

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#11193)
| Applies to    | 2.6+
| Fixed tickets | -

Commits
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9879ee6 Add note about invokable controller services