Fixed the section about getting services from a command
Linked the cookbook about DIC scopes
feature #3889 Fixed the section about getting services from a command (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
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Fixed the section about getting services from a command

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.3+
| Fixed tickets | #890

Hopefully this PR fixes one of the oldest errors of the Symfony documentation.

Commits
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8993beb Linked the cookbook about DIC scopes
0f3c218 Fixed the section about getting services from a command