Add new message to Geo troubleshooting doc

What does this MR do?

I ran into this error today, and coincidentally a customer had this problem today - ZD internal use only

If a user accidentally runs sudo gitlab-rake geo:set_secondary_as_primary on a primary node, we see a NoMethodError: undefined method `secondary?' for nil:NilClass error. Despite our docs saying to run this on the secondary node, this might catch out some people.

In my case, I ran the command twice on my secondary node, the first time it succeeded, but the second time it didn't, because the former secondary was now a primary.

Adding to the docs to save someone in the future.

Related issues

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