Add TrainState docstring with Optimisers API

The default TrainState docstring suggests using the version with the
Optimisers API, but it doesn't appear in the doc website. Add the
docstring manually to the website by adding the method to a @docs block.

docs/src/api/Lux/utilities.md

@@ -13,6 +13,7 @@ basic building blocks which can be seamlessly composed to create complex trainin
 
 ```@docs
 Training.TrainState
+Training.TrainState(::AbstractLuxLayer, ::Any, ::Any, ::Optimisers.AbstractRule)
 Training.compute_gradients
 Training.apply_gradients
 Training.apply_gradients!