[WCM] document the doctrine compiler pass

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (https://github.com/symfony/symfony/pull/7599)
| Applies to    | master, not sure about 2.2
| Fixed tickets | 

not sure if this should go to master or the 2.2 branch? the feature is not yet merged.