docs: document what AuthConfig.Auth is

[davidhsingyuchen]

PR enables folks to understand what Auth is without the need to trace code to:

cli/cli/config/configfile/file.go

Lines 228 to 229 in 65d3f78

	// decodeAuth decodes a base64 encoded string and returns username and password
	funcdecodeAuth(authStrstring) (string, string, error) {

Signed-off-by: Hsing-Yu (David) Chen davidhsingyuchen@gmail.com

[thaJeztah]

Thank you!

I want to double-check how this field is used, to be sure it's documented correctly as there's been some changes over time, and I'm not sure currently if this would always be username:password. These fields would be used to authenticate with different registries, and Authentication is not the same for all of them (which probably is one of the the reasons the field hasn't been documented so far).

I did a first search in history, but didn't completely finish that yet, but let me copy my notes here (in case I want to continue searching or someone else wants to);

• This type (in docker/cli) was taken from the engine's API types (done in Remove docker api dependency from cli/config #1642 to simplify some dependency issues).
• At some point the IdentityToken was introduced (Add support for getting token from login docker-archive-public/docker.engine-api#108), but I need to verify if that implementation actually was fully implemented all the way through. ISTR there were situations where the password field is used to store a token, and Add support for getting token from login docker-archive-public/docker.engine-api#108 (comment) also mentions some things about this. And, there's the RegistryToken field (which is confusing to have both here); moby/moby@8dce8e9
• The type moved around various times (e.g., Move more api types to api/types package moby/moby#18637, moby/moby@5b321e3, moby/moby@bb9da6b), which makes it a bit more difficult to get the full picture.