Improve documentation of optional flex-model and flex-context

[Flix6x]

I believe the function now should be more helpful in its docstring

• there is no fetching if there is no explicit flex model, but that only applies to the asset case of working.
• flex context and flex model being optional is not mentioned (for model starting 0.29 - we could mention that now already)
• how should the flex context and model even look like? Can we link to our docs?

Originally posted by @nhoening in #141 (review)