Document how we estimate users by transport by country

On https://metrics.torproject.org/userstats-bridge-combined.html?country=ar at the bottom it says "Even though bridges don't report a combination of clients by country and transport, it's possible to derive and graph lower and upper bounds from existing usage statistics. For further details see these questions and answers about user statistics."

But the further details link doesn't tell me how this graph derives lower and upper bounds from existing usage statistics.

Maybe it is already written up somewhere, and we can link to it?