Using {{doc}} on a page will transclude the text of the /doc/ sub-page, (if that page exists). If it does not, it will do nothing.

On Template:spoiler, the sub-page would be Template:spoiler/doc.

{{doc}}'s purpose is to make maintaining template documentation easier, while also separating the edit history of the documentation from the edit history of the template itself.

The /doc/ sub-page should be placed in Category: Externalized documentation (within a <noinclude> tag.)

<noinclude>[[Category: Externalized documentation]]</noinclude>