This template can be used on any other template page to transclude documentation to the page.
If a documentation subpage (/doc) already exists, the content of that page will be displayed (like in this case). Otherwise, it will show a short note about missing documentation combined with a link to the subpage to be written.
This is useful to reduce the amount of wikicode that is transcluded every time the template in question is used. Everything that does not need to be transcluded should be put on this subpage, including interwiki links.