document the namespace alias feature
feature #4069 document the namespace alias feature (dbu)

This PR was merged into the master branch.

Discussion
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document the namespace alias feature

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | [yes](https://github.com/symfony/symfony/pull/10853)
| Applies to    | 2.6
| Fixed tickets | -

Commits
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d753d0e document the namespace alias feature