additional parameters: `author`, `url`, and `subject`. The `subject`
parameter may also be filled with the user's comment about the change.
-Note: When the user edits an existing wiki page, the passed `content` will
-include only the lines that they added to the page, or modified.
+Note: When the user edits an existing wiki page, this hook is also
+passed a `diff` named parameter, which will include only the lines
+that they added to the page, or modified.
The hook should return `undef` on success. If the content is disallowed, it
should return a message stating what the problem is, or a function