SAML_USER_MODEL is not in documentation

[peterfarrell]

We are unable to find SAML_USER_MODEL in the documentation. Is this an unsupported feature and shouldn't be used? Or is it a documentation oversight?

[peppelinux]

It Is used as Django user model extension

I would like tò documenti if I'd have time for that

If i give you some examples May I count on your support for a PR about It?