[doc] Eventcallbacks with generic type parameter - wrong API documentation section

[hakenr]

EventCallback<ModalHidingEventArgs> HxModal.OnHiding renders in Parameters section instead of Event callbacks.
It seems that all the generic EventCallback<T> are rendered in such way.

This probably originated from EventCallback<TValue> ValueChanged parameters where we wanted them in Parameters section.
We can take one of these approaches:

1. Render ALL event-callbacks in Event callbacks section (incl. ValueChanged).
2. Create exception for XyChanged event callbacks and leave them as Parameters (part of @bind-Xy syntax). All the other event-callbacks (with or without generic parameter) will be rendered in Event callbacks section (these should start with OnXy prefix, but it is only a convention, not a rule).

@Harvey1214, my preference is 2, but you decide.