Document that rmail-output-to-rmail-file now writes in emacs-mule.

etc/NEWS

@@ -1326,6 +1326,17 @@ an envelope-from address different from user-mail-address.
 ** The variable mail-specify-envelope-from controls whether to
 use the -f option when sending mail.
 
+** The Rmail command `o' (`rmail-output-to-rmail-file') now writes the
+current message in the internal `emacs-mule' encoding, rather than in
+the encoding taken from the variable `buffer-file-coding-system'.
+This allows to save messages whose characters cannot be safely encoded
+by the buffer's coding system, and makes sure the message will be
+displayed correctly when you later visit the target Rmail file.
+
+If you want your Rmail files be encoded in a specific coding system
+other than `emacs-mule', you can customize the variable
+`rmail-file-coding-system' to set its value to that coding system.
+
 ** Changes to TeX mode
 
 *** The default mode has been changed from `plain-tex-mode' to