include an explanation in the docs of the flattened cargo movement structure and how this links to the column naming convention

[syed1992]

The naming convention is actually a very useful indicator on the legs of the cargo movement, but initially, this was unclear to me. We should explain in the docs that the number of total columns is a dynamic feature and depends on the number of event occurrences i.e. 3 sts events will mean the column 'events.cargo_sts_event.0' will go up to 3 etc.