dbworker: add forward-ref to update doc when columns are changed
Created by: asdine
Related to https://github.com/sourcegraph/sourcegraph/pull/33066#pullrequestreview-921694934
Test plan
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Created by: asdine
Related to https://github.com/sourcegraph/sourcegraph/pull/33066#pullrequestreview-921694934
Only added a comment