docs(subsUnion): note why legacy telegram subs are skipped, not errors

imports/startup/server/calcUnionAsync.js

@@ -29,6 +29,8 @@ const calcUnionAsync = async (subs, decorated) => {
           { units: 'kilometers', steps: 144 }
         ));
       } else {
+        // Expected for old telegram subscriptions predating the distance field
+        // (e.g. distance: null) — logged, not an error, safe to skip.
         console.info(`Wrong element to do union ${JSON.stringify(osub)}`);
       }
     } catch (e) {