Add mention of authors page variable to docs (#2451)

The `authors` variable so far has only been documented as something you can define in the front matter for use of feeds, but it also works in templates to define the authors of a page. (For practical use see: 6c293fa1a4 (diff-672b08946ef5cbc8db5c086bf50651b69b29e9d7be0708a1de7ded170b440e99))

This PR adds it to the documentation so that folks don't have to dig in commit histories to figure out it also works in templates.

docs/content/documentation/templates/pages-sections.md

@@ -22,6 +22,7 @@ date: String?;
 updated: String?;
 slug: String;
 path: String;
+authors: Array<String>;
 draft: Bool;
 // the path, split on '/'
 components: Array<String>;