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",
-"destpage", and "content". It should return the filtered content.
+"destpage", "content" and "fullpage". "fullpage" is a true value if,
+and only if, a full page's content is being filtered, e.g. as opposed
+to a directive parameter. It should return the filtered content.
### preprocess