Add: Add missing option

[Vimal-Raghubir]

Closes #590.
- What I did
I added the shutdown-timeout option to the dockerd configuration documentation.
- How I did it
Went to the required file and added the option with a description after the existing options.
- How to verify it
Preview changes made.
- Description for the changelog

Added missing option to dockerd configuration documentation.

- A picture of a cute animal (not mandatory but encouraged)

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "590-Add-missing-option" git@github.com:Vimal-Raghubir/cli.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[thaJeztah]

Thanks! Looks like you forgot to sign-off the commit; can you amend your commit (with the instructions that @GordonTheTurtle (our bot) gave above)?

[Vimal-Raghubir]

Hi @thaJeztah. I have signed off the commit and was wondering if you could kindly review this PR. Thank you.

[silvin-lubecki]

LGTM

[thaJeztah]

LGTM, thanks!