Improve High Availability upgrade docs

[svelle]

Currently the wording in the HA upgrade docs is a bit ambiguous and doesn't make some of the problems and limitations of the upgrade process very clear. After running a customer incident this week that revolved around this I think we need to look at them again and maybe rework them to be a bit more clear to first timers.

I'm going to take a first stab at this and put up a WIP pr once I have something written down.