The /proc/sequence module source

I don't think it is possible to keep all the articles updated and in sync with all the updates in the kernel. In the end, they have the date when they are posted and so refer to the state as per that date.

You say you are a complete beginner. I'd suggest you continue visiting LWN as a very good reference which will, I am sure, help you to learn and understand the Linux kernel code and keep up with the latest trends. However, for the exact API, I'd suggest you visit kernel documentation, best for the exact kernel version you are working with. For the example this article looks into, check Documentation/filesystems/seq_file.txt.