This template transcludes a template's documentation page into the template. Documentation is separated from the template code so that it can be edited separately from the template, such as when the template is protected or high-use. Documentation includes everything that is not part of the template code, including categories.
Usage
{{Documentation/Documentation}}
Categories
When adding categories to the documentation page which describe the template, use <includeonly> to prevent the documentation page from being categorized as well.