Ensure ff changelog docs is consistent with danger

What does this MR do and why?

Re: !157799 (comment 1973864047)

The flow chart reads flag is default: off -> remove flag -> keep old code -> no changelog, but the instruction above the flow chart says Changing the feature flag itself (flag removal, default-on setting) should have a changelog entry.. The 2 info are conflicting. Latter is consistent with the expectation of danger so we will update the flow chart.

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