[ClassLoader] [Deprecation] add deprecation message in class documentation #12600

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #12600
| License       | MIT
| Doc PR        | https://github.com/symfony/symfony/blob/2.7/UPGRADE-3.0.md#classloader

I'm not sure if this is enough, can you help me?
Can you please move the notice below the namespace declaration (see https://github.com/symfony/symfony/pull/12739/files)