Improve documentation for Pub/Sub and add split stream example

[richardhenry]

Thanks for implementing the split() method re: issue #57. I added some (hopefully helpful) documentation and improved the pub/sub documentation a little bit.

[richardhenry]

Btw I removed the regular_client.flushdb(FlushingMode::Sync).await? and pub_sub_stream.close().await? lines from the doctests because I thought they distracted from the documentation a little bit and the tests seem to pass without them. If you think I should add them back let me know.

[mcatanzariti]

The problem if you remove regular_client.flushdb(FlushingMode::Sync).await? from the doc test is that when you run all the doc tests in a row, remaining data from a previous test could make the test fail...

Removing the close should be ok though.

[richardhenry]

Amended my commit with that change