Add some in-code documentation

Author: Bar Weiss

select.go

@@ -468,6 +468,10 @@ func (sb *SelectBuilder) BuildWithFlavor(flavor Flavor, initialArg ...interface{
 			buf.WriteString(sb.offsetVar)
 		}
 	case Presto:
+		// There might be a hidden constraint in Presto requiring offset to be set before limit.
+		// The select statement documentation (https://prestodb.io/docs/current/sql/select.html)
+		// puts offset before limit, and Trino, which is based on Presto, seems
+		// to require this specific order.
 		if len(sb.offsetVar) > 0 {
 			buf.WriteLeadingString("OFFSET ")
 			buf.WriteString(sb.offsetVar)