[1.5.x] Fixed #20735 -- clarified ManyToManyField constructor doc
[1.5.x] Fixed #20735 -- clarified ManyToManyField constructor doc

Backport of ac223ff6d1 from master

ccn-2m/django,bliti/django-nonrel-1.5,alx-eu/django,hasadna/django,ccn-2m/django,bliti/django-nonrel-1.5,alx-eu/django,imtapps/django-imt-fork,imtapps/django-imt-fork,alx-eu/django,bliti/django-nonrel-1.5,ccn-2m/django,imtapps/django-imt-fork,ccn-2m/django,hasadna/django,alx-eu/django,hasadna/django