Updated documentation for pip install in zsh

Fixes jupyterlab#1209

Updated documentation to handle installation using `zsh` on Macs, which is now the default shell.

docs/source/users/index.md

@@ -119,6 +119,11 @@ If you are not using JupyterLab and you only want to install the Jupyter AI
 `jupyter-ai` depends on `jupyter-ai-magics`, so installing `jupyter-ai`
 automatically installs `jupyter-ai-magics`.
 
+*Note*: This installation step fails in a `zsh` environment because `zsh` uses square brackets for pattern matching. `zsh` is the default shell for Mac users since Catalina https://support.apple.com/en-ca/102360. The square brackets need to be escaped or the entire package name quoted. Please use the following install command instead:
+
+    $ pip install 'jupyter-ai[all]' 
+
+
 ### Minimal installation via `pip`
 
 Most model providers in Jupyter AI require a specific dependency to be installed