emphasize expire_on_commit in detached error docs
emphasize expire_on_commit in detached error docs

issues like #8082 suggest users are still not
fully aware of the need to set this parameter
when dealing with detached objects.

Change-Id: I6f389fdbe18b9c977bfb8188fc4732dbd56884d9

sqlalchemy/sqlalchemy,zzzeek/sqlalchemy