Fix wrong method in findTaggedServiceIds(), add example to docblock.
merged branch beberlei/TaggedServices (PR #7909)

This PR was submitted for the master branch but it was merged into the 2.1 branch instead (closes #7909).

Discussion
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[DependencyInjection] Fix method use + example in doc block

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

Fixes using a wrong method in ``findTaggedServiceIds`` and adds an example for the usage and showing the returned data structure as ``@return array`` is not very helpful, the datastructure being a bit unintuitive.

Commits
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ad2c955 [DependencyInjection] Fix wrong method in findTaggedServiceIds(), add example to docblock.