Update user guide to enable elyra[all] install on zsh

[leucir]

What changes were proposed in this pull request?

Update user guide to enable elyra[all] install on zsh

Fixes #2834

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[ptitzler]

I did a quick scan and found one more doc reference in https://github.com/leucir/elyra/blob/fix_doc_install/docs/source/user_guide/pipeline-components.md that we need to update

[akchinSTC]

@leucir - if youre busy i can update this for you and merge. thanks