ob-core.el: Improve org-babel-default-header-args docstring

* lisp/ob-core.el (org-babel-default-header-args): Provide an example
illustrating one benefit of using closures as default header
arguments.  Additionally, explain how to provide the same type of
header argument multiple times in the default alist.

lisp/ob-core.el

@@ -482,12 +482,14 @@ For the format of SAFE-LIST, see `org-babel-safe-header-args'."
 This is a list in which each element is an alist.  Each key
 corresponds to a header argument, and each value to that header's
 value.  The value can either be a string or a closure that
-evaluates to a string.  The closure is evaluated when the source
-block is being evaluated (e.g. during execution or export), with
-point at the source block.  It is not possible to use an
-arbitrary function symbol (e.g. \\='some-func), since org uses
-lexical binding.  To achieve the same functionality, call the
-function within a closure (e.g. (lambda () (some-func))).
+evaluates to a string.
+
+A closure is evaluated when the source block is being
+evaluated (e.g. during execution or export), with point at the
+source block.  It is not possible to use an arbitrary function
+symbol (e.g. 'some-func), since org uses lexical binding.  To
+achieve the same functionality, call the function within a
+closure (e.g. (lambda () (some-func))).
 
 To understand how closures can be used as default header
 arguments, imagine you'd like to set the file name output of a
@@ -504,7 +506,16 @@ this with:
 
 Because the closure is evaluated with point at the source block,
 the call to `org-element-at-point' above will always retrieve
-information about the current source block.")
+information about the current source block.
+
+Some header arguments can be provided multiple times for a source
+block.  An example of such a header argument is :var.  This
+functionality is also supported for default header arguments by
+providing the header argument multiple times in the alist.  For
+example:
+
+'((:var . \"foo=\\\"bar\\\"\")
+  (:var . \"bar=\\\"foo\\\"\"))")
 
 (put 'org-babel-default-header-args 'safe-local-variable
      (org-babel-header-args-safe-fn org-babel-safe-header-args))