Revive model inheritance documentation
|Reported by:||mtredinnick||Owned by:||nobody|
|Has patch:||no||Needs documentation:||no|
|Needs tests:||no||Patch needs improvement:||no|
So I don't forget: the model inheritance documentation seems to have been dropped during the docs merge. So we have to extract that out of the old models-api.txt. There was quite a bit of it, mostly all in one block towards the end of the file. Plus a couple of references earlier (e.g. in the related name section).