Here's a relatively recent example of it being used in US headline writing to mean an explanatory pamphlet -

"Republicans to Distribute Shutdown Explainer"

http://www.rollcall.com/news/republicans_to_distribute_shutd...

And here's another from The Washington Post, to mean the text of the article -

"Who is Daft Punk? A quick explainer for confused Grammy-watchers"

http://www.washingtonpost.com/blogs/style-blog/wp/2014/01/27...