changelog: add entry.

Author: kivikakk

changelog.txt

@@ -8,6 +8,9 @@ New APIs:
 
 Changed APIs:
 
+* `html::format_document`, `xml::format_document`, `cm::format_document` and
+  friends now take an `std::fmt::Write` as their `output` argument, instead of
+  an `std::io::Write`.
 * `html::collect_text` now returns a `String`.  `html::collect_text_append` is
   added if you still want to start with your own (`String`) buffer.
   * There was no particular reason for this populating a `Vec<u8>` instead of a