alternative to `exit` command: `CTRL-D`

@ostezer Before using the code notation for the EOF hotkey, the first thing I found was https://github.com/docker/docker/blame/master/docs/sources/reference/commandline/cli.md#L145 where the `CTRL-C` form is used. 
I searched through the docs some more now, and it doesn't seem like it's used consistently. 

@fredlf What's your (or Docker Inc's) opinion/preference? On the long run, a doc style guide would be great..

[My personal taste would be that all commands (written as words on the command line and hotkeys) should use the same formatting, `as code`. In most documentation on the web, commands and hotkeys are both formatted with a monospace font to better distinguish from prose.]