Document CML auto-generation #4544
Description
📚 Documentation
Currently it's quite difficult to work out that a .cml file can be recreated when running the tests if you know it should change, which leads to a temptation to do this manually (which is slow).
The Iris documentation doesn't appear to cover IRIS_TEST_CREATE_MISSING or --create-missing at all. This should be addressed in the Developers Guide (or it should be easier to find if it's in there, I haven't managed on several searches.