Clarify that Flux.params updates

I was confused when initally reading the overciew, because I though that `Flux.params` returned a touple of vectors. But it later becomes apparent that it actually returns another data-structure, because it updates when parameters change. I have added a single sentence to clarify this point. Edits on the wording/phrasing are very welcome.

Author: KronosTheLate

docs/src/models/overview.md

@@ -117,7 +117,7 @@ julia> parameters = Flux.params(predict)
 Params([[-0.99009055], [0.0]])
 ```
 
-These are the parameters Flux will change, one step at a time, to improve predictions. Each of the parameters comes from the `predict` model: 
+These are the parameters Flux will change, one step at a time, to improve predictions. Each time a parameter in your model changes, the `parameters` variable updates automatically. Each of the parameters comes from the `predict` model: 
 
 ```
 julia> predict.weight in parameters, predict.bias in parameters