eve/alert: add comment about string format

src/output-json-alert.c

@@ -257,6 +257,8 @@ void AlertJsonHeader(const Packet *p, const PacketAlert *pa, SCJsonBuilder *js,
         SCJbOpenObject(js, "context");
         const struct PacketContextData *json_info = pa->json_info;
         while (json_info) {
+            /* The string is valid json as it is validated by JANSSON
+               during parsing and included later via a format string */
             SCJbSetFormatted(js, json_info->json_string);
             json_info = json_info->next;
         }