Add documentation about service decoration
feature #3753 [DependencyInjection] Add documentation about service decoration (romainneutron)

This PR was submitted for the master branch but it was merged into the 2.5 branch instead (closes #3753).

Discussion
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[DependencyInjection] Add documentation about service decoration

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes (symfony/symfony#10600)
| Applies to    | 2.5+
| Fixed tickets | #3745

Commits
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578d2e2 [DependencyInjection] Add documentation about service decoration