docs: note --log-file does append

docs/content/docs.md

@@ -757,6 +757,8 @@ This can be useful for tracking down problems with syncs in
 combination with the `-v` flag.  See the [Logging section](#logging)
 for more info.
 
+If FILE exists then rclone will append to it.
+
 Note that if you are using the `logrotate` program to manage rclone's
 logs, then you should use the `copytruncate` option as rclone doesn't
 have a signal to rotate logs.