[LA-29914] Update documentation for teams_users#update endpoint

[rlarcombe]

Includes info about response options

[alexfiennes]

I would make the name of the default one be TeamUsers::Identity rather than Simple. We are not actually checking the value so you can technically use anything and it will fallback on the default, but I think it is better if we use the name that it actually corresponds to so that if in the future we do start checking the value then the docs are accurate?