Consider FromMultiSourceAttribute Necessity and Document It If It Helps

Version 3.1 sometimes had issues with request types being pulled from multiple locations (route, query, body).

[Nick Chapsas shows a solution to this problem here](https://youtu.be/SDu0MA6TmuM?t=647). The MultiSourceAttribute.

I'm not sure if the problem still exists in v4, but if it does we should show how to use this attribute to address it.

References:

- https://stackoverflow.com/questions/45495432/asp-net-core-mvc-mixed-route-frombody-model-binding-validation
- https://www.seeleycoder.com/blog/mixed-model-binding-aspnetcore/

Start by checking whether v4 still reproduces the mixed route, query, and body binding problem described in the issue and linked references. Review the MultiSourceAttribute entry point; done means either documenting its usage for the current behavior or recording that it is no longer needed.

Written by the indexing model from the issue text.