docs: reference templates in top-level passwd mgmt

It's important for people to understand that the password manager
feature is an extension/implementation of the existing templating
capabilities of Chezmoi.

assets/chezmoi.io/docs/user-guide/password-managers/index.md

@@ -1,6 +1,29 @@
-# Password manager integration
+# Password Manager Integration
 
-Template functions allow you to retrieve secrets from many popular password
-managers. Using a password manager allows you to keep all your secrets in one
-place, make your dotfiles repo public, and synchronize changes to secrets
-across multiple machines.
+Using a password manager with Chezmoi enables you to maintain a public
+dotfiles repository while keeping your secrets secure. Chezmoi extends its
+existing [templating capabilities](../templating.md) by providing password
+manager specific _template functions_ for many popular password managers.
+
+When Chezmoi applies a template with a secret referenced from a password
+manager, it will automatically fetch the secret value and insert it into the
+generated destination file.
+
+## Example: Template with Password Manager Integration
+
+Here's a practical example of a `.zshrc.tmpl` file that retrieves an OpenAI API
+key from 1Password while maintaining other standard shell configurations:
+
+```zsh
+# set up $PATH
+# …
+
+# OpenAI API Key retrieved from 1Password
+export OPENAI_API_KEY='{{ onepasswordRead "op://Personal/openai-api-key/password" }}'
+
+# set up aliases and useful functions
+```
+
+In this example, the `OPENAI_API_KEY` is retrieved from a 1Password vault
+named `Personal`, specifically from an item called `openai-api-key` in the
+`password` field.