change fpath instructions to be more broad (#936)

Co-authored-by: Joyce Fee <[email protected]>

modules/reference/pages/rpk/rpk-generate/rpk-generate-shell-completion.adoc

@@ -34,26 +34,38 @@ rpk generate shell-completion bash > /etc/bash_completion.d/rpk
 
 == Zsh
 
-To enable autocompletion in any zsh session for any user, run this once:
+To enable autocompletion in any zsh session for any user, follow these steps:
 
-[,bash]
+Determine which directory in your `$fpath` to use to store the completion file.
+
+You can inspect your `fpath` by running:
+[,zsh]
 ----
-rpk generate shell-completion zsh > "${fpath[1]}/_rpk"
+echo $fpath
 ----
 
-You can also place that command in your `~/.zshrc` to ensure that when you update
-`rpk`, you update autocompletion. If you initially require `sudo` to edit that
-file, you can `chmod` it to be world writeable, after which you will always be
-able to update it from `~/.zshrc`.
+Choose one of the directories listed. For example, if `/usr/local/share/zsh/site-functions` is present in your `fpath`, you can place the `_rpk` completion file there:
 
-If shell completion is not already enabled in your zsh environment, also
-add the following to your `~/.zshrc`:
+[,zsh]
+----
+rpk generate shell-completion zsh > /usr/local/share/zsh/site-functions/_rpk
+----
 
+If the directory you chose is not already in `fpath`, add it to your `.zshrc`:
 [,zsh]
 ----
-autoload -U compinit; compinit
+fpath+=(/usr/local/share/zsh/site-functions)
 ----
 
+Finally, ensure that `compinit` is run. Add (or verify) the following in your `.zshrc`:
+
+[,zsh]
+----
+autoload -U compinit && compinit
+----
+
+After restarting your shell, `rpk` completion should be active.
+
 == Fish
 
 To enable autocompletion in any `fish` session, run: