Add alert to "avoid persisting detached entities" (#8109)
Add alert to "avoid persisting detached entities" (#8109)

The alert is hidden into the code of ``EntityManager``,
while it's useful to be mentioned in the documentation.
doctrine/doctrine2,Sitetheory/doctrine2,bigfoot90/doctrine2,doctrine/doctrine2,bigfoot90/doctrine2,Sitetheory/doctrine2