2.x: explain null plugin returns better

[akarnokd]

When an onSubscribe hook is installed and returns a null consumer, the current error report says:

ObjectHelper.requireNonNull(observer, "Plugin returned null Observer");

At least an "a" is missing but perhaps it would be better to word it more detailed:

"The RxJavaPlugins.onSubscribe hook returned a null Observer. Please check the handler provided to RxJavaPlugins.setOnObservableSubscribe for invalid null returns. Further reading: https://github.com/ReactiveX/RxJava/wiki/Plugins"

Of course, all reactive types should be updated and adapted to the appropriate types and method names.

[logan-firefly]

I'm inexperienced but I would be a happy to try and help with this issue.

[akarnokd]

Closing via #6175.