Update RouterFunction docs to reflect the way they're looked up

[JWThewes]

I'm building a larger application using RouterFunctions. What I noticed relative quickly after starting is that I built a configuration class which is depended from nearly all the services in my application. This is definitely something we all want to avoid.
So I reached out to Stéphane Nicoll & Phil Webb and was told that I can easily split the RouterFunctions into several BeanDefinitions and they will be reconciliated for me during startup. (https://twitter.com/snicoll/status/950636452232167425)

This should be reflected in the docs so people don't start building massive Configuration Classes which depend on nearly everything.

[snicoll]

I've also review the core Spring Framework documentation and I think we should improve the Java configuration section. I've raised SPR-16360.