Clarify makeFieldLabels prefix requirements

[adamgundry]

Talking to @edsko I think the current distinction between makeFieldLabels and makeFieldLabelsNoPrefix is a bit non-obvious. In particular, if a user uses makeFieldLabels on a datatype that has no prefixes, it will silently succeed but generate no LabelOptic instances.

I think we should:

• Document in the Haddocks that the prefixes are required, and explain what to do if different prefix rules are preferred.
• Have makeFieldLabels emit a warning if none of the fields have the expected prefix.