Add doc on Icedove reply_header_

wiki/src/todo/Return_of_Icedove__63__.mdwn

@@ -52,7 +52,8 @@ Things to be checked / researched
   paper](http://bit.ly/qDZm7C); some of its recommendations need to be
   discussed: [[!tag todo/discuss]]
   - do we really want to encourage people to use PGP-inline?
-  - `mailnews.reply_header_authorwrote = %s` is uncommon
+  - `mailnews.reply_header_authorwrote = %s` is uncommon, see [the doc on
+    reply_header_](http://kb.mozillazine.org/Reply_header_settings)
 
 found answers
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