Update the documentation and debugging section with info about new channel-default-controller

[akashrv]

I'd actually update the Release Notes seciton, since it has user visible changes. Also, we should make sure to update documentation with this information and probably update the debugging also. Not as part of this PR, but we should probably create an issue to track or put out a PR to update those.

Originally posted by @vaikas-google in #843 (comment)

[akashrv]

/assign

[mikehelmick]

this goes away w/ channel CRDs

/close

[knative-prow-robot]

@mikehelmick: Closing this issue.

Details

In response to this:

this goes away w/ channel CRDs

/close

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