org-manual: Document "sep" header argument

* doc/org-manual.org (Results of Evaluation): Document "sep" header
  argument.

doc/org-manual.org

@@ -17190,6 +17190,11 @@ default behavior is to automatically determine the result type.
      uses the generated file name for both the "link" and
      "description" parts of the link.
 
+     #+cindex: @samp{sep}, header argument
+     By default, Org assumes that a table written to a file has
+     TAB-delimited output.  You can choose a different separator with
+     the =sep= header argument.
+
 *** Format
 :PROPERTIES:
 :UNNUMBERED: notoc