Retractions #17982
Description
We still haven't added retractions to Beam, even though they're a core part of the model. We should document all the necessary aspects (uncombine, reverting DoFn output with DoOvers, sink integration, source-level retractions, etc), and then implement them.
Imported from Jira BEAM-91. Original Jira may contain additional context.
Reported by: takidau.