c_resource_count: use displays for some category codes where possible

[mikix]

Like for DiagnosticReport - the two US Core profiles together have two different systems in use - but only four total codes. Three of which are loinc and thus not really for humans.

Instead, for cases like that where it would be easy to map to strings (like we do for race & ethnicity), we should use pleasant strings.

Should do a quick scan for other simple cases too, but at least DiagReport is one.