Registry doc: updating Kafka output and few other cosmetics

[nachocano]

Fixes knative/eventing#929

Proposed Changes

• This is a follow up on the other registry PR, as I updated the Kafka code, it outputs another source value.
• Cosmetic changes to the rest of the doc.

[nachocano]

/assign @samodell
Sam this is a follow up PR. It seems more changes than it actually is. It should be an easy approve.
I updated the kafka output, and few other cosmetics in the text. Sockpuppet I think will take care of the formatting once this lands.

[nachocano]

@samodell done with the changes. I need to focus on some important code fixes.
IMO this can go in, and then we can keep on iterating. What do you think?

[samodell]

Agreed! Thanks, @nachocano !

/lgtm
/approve

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