call out restrictions on Var in code blocks (#18393)

docs/pages/contributing/documentation/reference.mdx

@@ -512,6 +512,14 @@ $ curl https://<Var name="proxy"/>/webapi/ping
 
 </Details>
 
+<Admonition type="tip">
+
+The `Var` component only works in [code blocks](#code-blocks) using the `code`
+syntax (e.g. <code>```code</code>), and only for "command" lines (beginning
+with `$`). The component will work without caveats outside of code blocks.
+
+</Admonition>
+
 When a user updates the value of an in-page variable, the values of any `Var`
 components with the same `name` property update automatically. When you copy a
 `code` snippet using the copy button, the values of any in-page variables within