Document when and how to test RC generation

Opening as a corrective action for production#3652 (closed) although this isn't the only time we've recently made a change to RC tagging that involved testing.

We should document:

• the steps to take to test RC tagging
• a recommendation for tagging different releases based on risk (e.g. a patch release is less risky than a monthly release because there are fewer changes and no deadline to meet)
• types of tooling change that need careful testing against release preparation