Document how it works in multi-table inheritance

[pakal]

When doing multi-table inheritance in Django, it works fine to have "changesets" manager only on the parent table, but each of the children models must specify the "track_fields" and "track_related" in its own Meta attribute.

Possibly copying+extending "ParentClass_meta.track_fields" to be DRY

Maybe it's worth noting it somewhere in docs, in case other people stumble on this (field changes were not tracked, as a result).