improve docs

internal/selectors.go

@@ -16,12 +16,15 @@ func And(a, b Selector) Selector {
 	}
 }
 
+// OnlyNature will only select records with the given Nature n (G01, G20, etc...).
 func OnlyNature(n Nature) Selector {
 	return func(r Record) bool {
 		return r.Nature() == n
 	}
 }
 
+// OnlyAssetCountry will only select records with the given ISO code c (620 for Portugal, 196 for
+// Cyprus, etc...).
 func OnlyAssetCountry(c int64) Selector {
 	return func(r Record) bool {
 		return r.AssetCountry() == c