grpc-js: Add comment about stopKeepalivePings usage

packages/grpc-js/src/subchannel.ts

@@ -384,6 +384,11 @@ export class Subchannel {
      * sending pings should also involve some network activity. */
   }
 
+  /**
+   * Stop keepalive pings when terminating a connection. This discards the
+   * outstanding ping timeout, so it should not be called if the same
+   * connection will still be used.
+   */
   private stopKeepalivePings() {
     clearInterval(this.keepaliveIntervalId);
     clearTimeout(this.keepaliveTimeoutId);