Auto merge of #1347 - cantsin:helper-class-docfix, r=acorncom
Auto merge of #1347 - cantsin:helper-class-docfix, r=acorncom

Clarify class-based helper format documentation.

Fixes https://github.com/emberjs/guides/issues/1318

Also-by: Dig <db0f7daf3379120050645d138fd7de2e51a6703b@locusenergy.com>

bmac/guides,bmac/guides,HeroicEric/guides,boztek/guides,nicobot/guides,knownasilya/guides,knownasilya/guides,boztek/guides,ubuntuvim/guides,HeroicEric/guides,nicobot/guides,skaterdav85/guides,skaterdav85/guides,davidpett/guides,MarkMT/guides,jkarsrud/guides,jkarsrud/guides,skaterdav85/guides,ubuntuvim/guides,HeroicEric/guides,tleskin/guides,davidpett/guides,jkarsrud/guides,jkarsrud/guides,nicobot/guides,ubuntuvim/guides,MarkMT/guides,ubuntuvim/guides,tleskin/guides,bmac/guides,MarkMT/guides,knownasilya/guides,MarkMT/guides,davidpett/guides,knownasilya/guides,boztek/guides,davidpett/guides,tleskin/guides,skaterdav85/guides,bmac/guides