Added limitations in 'Managing Common Dependencies with Parent Services'...

This PR adds a caution in 'Managing Common Dependencies with Parent Services' page.

| Q             | A
| ------------- | ---
| Doc fix?      | yes (Improvement on existing docs)
| New docs?     | no
| Applies to    | all 
| Fixed tickets | -

Ref: [Source code](https://github.com/symfony/DependencyInjection/blob/master/Compiler/ResolveDefinitionTemplatesPass.php#L79),   [Issue](https://github.com/symfony/symfony/issues/4620)

Note: In 2.0 branch, [commit](https://github.com/symfony/symfony-docs/commit/2e7d5aa1eee8b31cf0f6c98aca366aabdfd54ef1) (Fixing a bad merge conflict) hasn't been applied yet. As this PR was built on top of 2.0 branch, still this contains the conflicts text. I have another [PR](https://github.com/symfony/symfony-docs/pull/2501) in which I have applied that commit manually.