Dev Note: A plan to improve the documentation on Buffer plugins

[fujimotos]

In this issue, I'd briefly discuss the plan to improve the v1.0 article on Buffer plugins.

Current status

As you can see in the article linked above, the current documentation on buffer plugins
has been under construction for a while. Since there seem some general confusions
about how buffers works, I think it's worth updating.

What should we do?

Below is my rough plan to improve the current documentation:

1. Add an introductory description on how buffers work. We need something concise to
   expain how buffers work *from a user perspective*. For example, I think it'll help admins
   a lot if the failure modes of buffers are explained in a clear way.

2. Add annotated configuration examples. For now, there is no concrete configuration
   example in the buffer article. We can provide well-annotated examples so that users
   can base their work on them.

3. Add a few technical tips. For example. there is no general description on
   how an exponential backoff works and it apparently confuses some users.
   We can avoid this kind of issues by adding a brief note to the documentation.

I'd lay out more details in the subsequent comments of this issue.

Next step

I'll work on this issue in the next several weeks;
A first batch of pull requests (mostly "small fix" kinds) will land in a few days,