org-manual.org: Modify file-desc header argument to match action

* doc/org-manual.org (Results of Evaluation): Correct description of
how file-desc behaves when no value is given.

TINYCHANGE

doc/org-manual.org

@@ -17345,9 +17345,8 @@ default behavior is to automatically determine the result type.
 
   #+cindex: @samp{file-desc}, header argument
   The =file-desc= header argument defines the description (see
-  [[*Link Format]]) for the link.  If =file-desc= has no value, Org
-  uses the generated file name for both the "link" and
-  "description" parts of the link.
+  [[*Link Format]]) for the link.  If =file-desc= has no value, the
+  "description" part of the link will be omitted.
 
   #+cindex: @samp{sep}, header argument
   By default, Org assumes that a table written to a file has