DDC-1201: DQL Example about count(*) related entity is wrong

[doctrinebot]

Jira issue originally created by user @beberlei:

DQL Example about count(*) related entity is wrong

[doctrinebot]

Comment created by gedrox:

Also similar DQL in documentation URL http://www.doctrine-project.org/docs/orm/2.1/en/reference/dql-doctrine-query-language.html#pure-and-mixed-results fails parsing stage:

$dql = "SELECT u, 'some scalar string', count(u.groups) AS num FROM User u JOIN u.groups g GROUP BY u.id";

with error

  [Doctrine\ORM\Query\QueryException]                                                                                                                         
  [Semantical Error] line 0, col 40 near 'localizations)': Error: Invalid PathExpression. StateFieldPathExpression or SingleValuedAssociationField expected. 

Should be "count(g.id)" instead of "count(u.groups)".

[lcobucci]

Fixed by #6212