Improve description of comma delimited structure

[cjrace]

Is your feature request related to a problem? Please describe.
Comment on dfe-analytical-services/dfe-published-data-qa#129 that the guidance we point to might not help users who aren't familiar with the terminology to know what to do.

Describe the solution you'd like
An improved description of comma delimited structure and how delimited structures work / how to save files in that format.

Describe alternatives you've considered
None

Additional context
Related section beneath around saving as UTF-8 format. If either this header or that header changes we'll need to make sure to update the links in the data screener as we currently link to both sections a lot

[jen-machin]

Analysts Guide: improve description of comma delimited structure