Clarifications and improvements to Kafka manual testing documentation (…

…#422)

* Address feedback from #290 
* Add note about automated Kafka tests, which didn't exist at time the document was first written
* Fix spelling errors

Fixes #322

Authors:
  - David Gardner (https://github.com/dagardner-nv)
  - Michael Demoret (https://github.com/mdemoret-nv)
  - Pete MacKinnon (https://github.com/pdmack)

Approvers:
  - Michael Demoret (https://github.com/mdemoret-nv)
  - Pete MacKinnon (https://github.com/pdmack)

URL: #422