make the requirement that parameter names are needed when using generative slots more explicit in the documentation

[atinsood]

when using generative slots like in the example below

@mellea.generative
def classify_sentiment(text: str) -> Literal["positive", "negative"]:
  """Classify the sentiment of the input text as 'positive' or 'negative'."""

following code does not work,

label = classify_sentiment(m, "label this sentiment")

while this code works

label =  classify_sentiment(m, text="label this sentiment")

it seems like you need to provide the parameter names when calling the generative slots, but its not explicitly pointed out in the documentation. It would be useful to call this out as a requirement in the documentation as well have a comment in the examples that this is expected.

[jakelorocco]

Currently, you can actually pass args positionally; it's just that all the Mellea args need to be passed positionally as well.

However, I am introducing a change that will force the parameters from the decorated function to be keyword arguments. This will be explicitly enforced when calling an @generative function and the docstring will be fixed as well.