[1.5.x] Fixed #19444 -- Clarified IGNORABLE_404_URLS documentation
[1.5.x] Fixed #19444 -- Clarified IGNORABLE_404_URLS documentation

Backport of 53b879f045 from master.

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