add a commit-rule precision when applying patches discussed on the mailing list

Originally committed as revision 8744 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/ffmpeg-doc.texi

@@ -1547,6 +1547,9 @@ please use av_log() instead.
    archived you should add some SPAM protection to the email address. Send an
    answer to ffmpeg-devel (or wherever you got the patch from) saying that
    you applied the patch.
+@item
+   When applying patches that have been discussed (at length) on the mailing
+   list, reference the thread in the log message.
 @item
     Do NOT commit to code actively maintained by others without permission.
     Send a patch to ffmpeg-devel instead. If noone answers within a reasonable