Board Thread:Support Requests - Getting Technical/@comment-25982240-20151119223022/@comment-1757994-20151120001753

The Half-Life wiki page to which you pointed just has two hard-coded tables at the top of the page, transcluded from templates, one with a spoiler warning and one with a clean-up notice.

For example, they first created Template:Spoiler, and then put at the top of the page.

You can choose to do it the same way, or just put something like  Some of the computers mentioned on this page might have never existed so blablabla... at the top of the page and add whatever styling (borders, different background color, bold font, etc.) you want.