Document examples of how to use the PropagatedContext API with RxJava

[jeremyg484]

Issue description

We are doing what we can to pass any available values in the PropagatedContext along to RxJava flows, but the ability to do so automatically for all cases is limited due to RxJava not having integrated support for the Micrometer Context Propagation SPI. There are inevitably going to be some cases where a user of RxJava with Micronaut may need to directly use the PropagatedContext API if they want context values to be available within their reactive flow. Some general documentation examples should be provided that shows how to do this, and some more general tests exercising these techniques should be added to the module.