Auth docs: better explain userSignupFields and SignupForm additionalFields and their relationship

[Martinsos]

What I can't find in the docs is the info on what happens with those additional fields, defined via additionalFields, for the SignupForm (Auth UI). I guess they get passed along the other fields, like email, so if somebody is going to use them, ok. But we never explain how to use them, because the only thing that can use them is our default signup action that is being used by Auth UI? Is there a scenario where user will actually want to access them? If so, how can they do so? If not, who is using them exactly? Feels like this is not explained well.

Also, it seems to me like there is some kind of assumed relationship between userSignupFields and SignupForm additionalFields, but I don't think it is completely clear. Do the two have to be used together? Can additionalFields be used on their own? Can userSignupFields be used on their own?

Also, I learned that additionalFields make sense only for username & pass or email & pass, but that was not clear to me from the docs at all.

Related Discord convo: https://discord.com/channels/686873244791210014/1225671304313704508/1225671304313704508 .

[Martinsos]

Related to #1710 .