Document dind and cli variants

[yosifkit]

This also adds a section about docker-library/docker#559

Refs:

• Restrict channel aliases to 19.03 docker#179
• Update docker official-images#9253 was the PR that finally removed the stable and test aliases

[tianon]

Some very minor rewording / reorganization for your consideration (trying to make the "what's the action?" more obvious):

Suggested change

	**Note:**: The `*stable`, and `*test` tags have been deprecated since June 2020 (see [docker-library/docker#179](https://github.com/docker-library/docker/pull/179)) and were no longer updated in December 2020 when Docker 20.10 was released. The alternative to `stable` is to use `latest` (there was no difference when it was deprecated). You can also use the `cli`, and `dind` tags to get the latest Docker CLI or engine, respectively. The alternative to the `test` tags is to use the `rc` tag or a tag with `-rc` in it. The `rc` tag is only updated when there is an active pre-release and will not point to the same thing as `latest`.
	**Note:** The `docker:stable`, `docker:test`, and related "channel" tags have been deprecated since June 2020 (see [docker-library/docker#179](https://github.com/docker-library/docker/pull/179)) and have not been updated since December 2020 (when Docker 20.10 was released). Suggested alternatives:
	- `docker:stable` ⏩ `docker:latest` (aliases to the same image at the time of deprecation), `docker:X.Y`, etc
	- `docker:test` ⏩ `docker:rc`, `docker:X.Y-rc`, `docker:rc-cli`, etc (only updated when there is an active pre-release; will not point to the same thing as `latest`)