Add documentation for new footnote command.

doc/org.texi

@@ -1569,6 +1569,7 @@ s   @r{Sort the footnote definitions by reference sequence.  During editing,}
     @r{Org makes no effort to sort footnote definitions into a particular}
     @r{sequence.  If you want them sorted, use this command, which will}
     @r{also move entries according to @code{org-footnote-section}.}
+r   @r{Renumber the simple @code{fn:N} footnotes.}
 n   @r{Normalize the footnotes by collecting all definitions (including}
     @r{inline definitions) into a special section, and then numbering them}
     @r{in sequence.  The references will then also be numbers.  This is}