[release-1.13] Document how to set Kafka Topic config options

[knative-prow-robot]

This is an automated cherry-pick of #5854

[knative-prow[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Cali0707, knative-prow-robot

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• ~~docs/eventing/OWNERS~~ [Cali0707]

Approvers can indicate their approval by writing /approve in a comment Approvers can cancel approval by writing /approve cancel in a comment

[ReToCode]

Seems like this could go in, right?

[Cali0707]

/unhold