Clarify what ehrQL output formats refer to

[rebkwok]

Raised in Slack

ehrQL docs describe supported output formats but don't make it clear that this applies to highly sensitive outputs. Moderately sensitive outputs (i.e. those that can be viewed on L4/in Airlock and requested for release) have different supported formats. In some cases these are sort of contradictory - for example, we recommend csv.gz for ehrQL outputs, but we don't allow it as a format for release (csv is used for tables for release).

The ehrQL docs should clarify the type of output they're referring to.