CDC: Document backfill behavior

[jseldess]

Jesse Seldess commented:

PR: cockroachdb/cockroach#43037

From release notes:

[Changefeeds](../v19.1/create-changefeed.html] now emit backfill row updates for a dropped column when the table descriptor drops that column. [#43037][#43037) {% comment %}doc{% endcomment %}

Jira Issue: DOC-423

[mwang1026]

Coopting this doc to document when we backfill vs. when we don't

[mwang1026]

@amruss @mwang1026 add some detail here

[jseldess]

@amruss, @mwang1026, should we keep this open? If so, we might want to remove the 19.1 milestone. This issue was originally tied to a release note for 19.1 that we never documented.

[mwang1026]

I think we keep but remove the milestone

[amruss]

Agreed ^

[github-actions]

We have marked this issue as stale because it has been inactive for
18 months. If this issue is still relevant, removing the stale label
or adding a comment will keep it active. Otherwise, we'll close it in
10 days to keep the issue queue tidy. Thank you for your contribution
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[exalate-issue-sync]

Kathryn Hancox (kathancox) commented:
This is no longer relevant or on a version we support.