`login` documentation strange headlines

[thomas-riccardi]

Description

The login markdown documentation has strange headlines.

https://docs.docker.com/engine/reference/commandline/login/

ddadd3d mass standardized the
formatting, with some errors, notably in login.md:

• Logging out headline is wrong (it's just that docker logout is a prerequisite to the Usage documented in the following lines): ddadd3d#diff-c033ce901f31496d94f0e354c81a9cdbR139
• some headlines were shifted one level but not their own nested headlines
• nested Usage headlines were removed, making the surrounding text less comprehensible

I have a PR coming for this issue.

I am not sure if we want to restore the nested Usage headlines, should we?