WIP Feature flag rollout docs: correct OnByDefault and removal examples
The change I made in 938a7544 (Feature flag rollout docs: add examples of Go/Ruby code removal, 2021-02-25) didn't make much sense. The link to the "on by default" didn't actually link to such a MR.
This would be ideal, but in !3033 (merged) I made the stupid mistake of mixing up "delete" and "create" in the subject.
So let's have this wait, will rewrite this once I have UserUpdateBranch landed with something more sensible I can link to as an example.