Added missing UniqueEntity ignoreNull documentation

The ignoreNull options was added to the UniqueEntity constraint on 6 Aug 2012, but there was no documentation created for it. The PR with the code changes was merged after the release of SF 2.0


| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes (symfony/symfony#5015)
| Applies to    | 2.1+
| Fixed tickets | N/A

Merge commit ref: symfony/symfony@0ccda3877538367c5aa73f7c0a557e643cfb2c70