Mention the id variable for the anchor link template

Closes #727

Author: Keats

docs/content/documentation/content/linking.md

@@ -33,7 +33,7 @@ This option is set at the section level: the `insert_anchor_links` variable on t
 
 The default template is very basic and will need CSS tweaks in your project to look decent.
 If you want to change the anchor template, it can easily be overwritten by
-creating a `anchor-link.html` file in the `templates` directory.
+creating a `anchor-link.html` file in the `templates` directory which gets an `id` variable.
 
 ## Internal links
 Linking to other pages and their headings is so common that Zola adds a