Document adding a buffer before transactional producer

[seglo]

Short description

Document adding a buffering stage before the transactional producer so that some amount of messages can still be processed by user business logic while a commit is occurring in the producer stage (and back-pressuring).

This may help to improve throughput of transactional workflows in some cases.

[jrudolph]

Maybe it would be enough to document the implications of the transactional behavior on the user stream, instead? And add a snippet that people can experiment with a buffer themselves?

[seglo]

Good idea. We have enough of a maintenance burden with transactions as it is.

[ennru]

Let's try it in the benchmarks. (not that they'd have a lot of business logic, ofc)