completion/zsh: Allow multiple volumes of 'volume rm'

[jo-so]

It is possible to give more than one volume for docker volume rm.

---

- How to verify it

1. Create multiple volumes for i in aaa bbb ccc; do docker run -v $i:/foo debian /bin/true; done
2. Start a Zsh with the new completion file
3. Type docker volume rm aaa and hit TAB. It should show bbb and ccc instead of no more arguments

- Human readable description for the release notes

[codecov-commenter]

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[thaJeztah]

Thanks for contributing!

Note that we moved away from using the manually written zsh completion scripts, and now use completion scripts that are generated from source code. We keep the manually written script in the repository (for now), but may remove those at some point.

To install the generated completion scripts, you can follow the instructions as output by the docker completion command for your shell; in this case, for zsh;

docker completion zsh --help

Which will output something like;

Usage:  docker completion zsh [flags]

Generate the autocompletion script for the zsh shell.

If shell completion is not already enabled in your environment you will need
to enable it.  You can execute the following once:

	echo "autoload -U compinit; compinit" >> ~/.zshrc

To load completions in your current shell session:

	source <(docker completion zsh)

To load completions for every new session, execute once:

#### Linux:

	docker completion zsh > "${fpath[1]}/_docker"

#### macOS:

	docker completion zsh > $(brew --prefix)/share/zsh/site-functions/_docker

You will need to start a new shell for this setup to take effect.

Options:
      --no-descriptions   disable completion descriptions

[thaJeztah]

LGTM