The difference between format and sanitize is that sanitize only acts on
the page body, while format can modify the entire html page including the
-header and footer inserted by ikiwiki, the html document type, etc.
+header and footer inserted by ikiwiki, the html document type, etc. (It
+should not rely on always being passed the entire page, as it won't be
+when the page is being previewed.)
The function is passed named parameters: "page" and "content", and
should return the formatted content.