docker login reference can use some TLC

[thaJeztah]

Description

https://github.com/docker/cli/blob/6273e65ceedc8544765ddc940a4a6dc7c2a2a491/docs/reference/commandline/login.md

I noticed there's some quirky wording about "must be root" and "sudo" on that page that even refers to docker-machine; looks like we need to clean up some of that.

I should add that there's SOME value to some of that information, but perhaps something to put in a more general location; if memory serves me well, some of it was written because users were not familiar with sudo behavior; some of that resulting in "HOME" not being "HOME" (and thus a different ~/.docker/config.json being used than they expected)

With the work on OAUTH; there's probably some additional information we can add as well (interactive vs non-interactive)