manual: Clarify behavior of :file-desc

* doc/org-manual.org (Results of Evaluation): Distinguish between
behavior when the :file-desc header argument is not present and when
it has no value.

The current handling of the :file-desc argument is to omit the
description if the header argument isn't present and to use the :file
value as the description if :file-desc is present without a value.
This matches the original intention [*].

[*] https://orgmode.org/list/87vclky211.fsf@med.uni-goettingen.de

Reported-by: Matt Huszagh <huszaghmatt@gmail.com>
  https://orgmode.org/list/87sgbzl6uy.fsf@gmail.com

doc/org-manual.org

@@ -17348,8 +17348,9 @@ 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, the
-  "description" part of the link will be omitted.
+  [[*Link Format]]) for the link.  If =file-desc= is present but has no value,
+  the =file= value is used as the link description.  When this
+  argument is not present, the description is omitted.
 
   #+cindex: @samp{sep}, header argument
   By default, Org assumes that a table written to a file has