This is a wrapper for template categories, which are placed to the template's documentation page. Unlike the legacy code with the {{sandbox other}} template, this wrapper also suppresses incorrect categorization of a doc page that transcludes another doc page.
Usage edit
Put the following code to the end of a template documentation page:
{{Template categories|
[[Category:Cat1]]
[[Category:Cat2]]
...
}}
Unless the transcluding page is a /doc or /sandbox subpage, it will be placed into the specified categories. Surrounding the code with <includeonly> tags is not necessary, but it will not harm.