hook(type => "filter", id => "foo", call => \&filter);
Runs on the raw source of a page, before anything else touches it, and can
-make arbitrary changes. The function is passed named parameters `page` and
-`content` and should return the filtered content.
+make arbitrary changes. The function is passed named parameters "page",
+"destpage", and "content". It should return the filtered content.
### preprocess