Explain `adq_per_quantity` in a comment.
Explain `adq_per_quantity` in a comment.

Addresses review feedback from @inglesp

ebmdatalab/openprescribing,ebmdatalab/openprescribing,annapowellsmith/openpresc,annapowellsmith/openpresc,annapowellsmith/openpresc,ebmdatalab/openprescribing,ebmdatalab/openprescribing,annapowellsmith/openpresc