Document that types cannot depend on argument values

[gvanrossum]

We don't have dependent types. This came up in python/typeshed#521. Maybe we could add an example of typical code using itertools.product(), and how it could be typed.

[nanjekyejoannah]

@gvanrossum I would love to work on this if no one is atm.

[emmatyping]

@nanjekyejoannah Feel free to do so!

[ilevkivskyi]

We don't have dependent types.

Currently we do, and this is documented, so this issue can be closed.