Added documentation for errorPath option of UniqueEntity constraint

I have added documentation for `errorPath` option of the `UniqueEntity` constraint. But I have only added configuration examples for *yaml* and *annotations*, because I have not used *xml* or *php* configuration before. Maybe somebody else could add those examples ;-)

| Q                 | A
| ------------------- | ---
| Doc fix?        | no
| New docs?    | yes (commit https://github.com/symfony/symfony/commit/ac7875569ae23d3e9df2f2cb1bd45d2a71de7441)
| Applies to     | 2.1+
| Fixed tickets | no