org-yank: Correct docstring

* lisp/org.el (org-yank): Update docstring for change in default value
  of org-yank-adjusted-subtrees.

The default value was changed to nil in 2949012 (Change default values
of some variables., 2009-02-01).

Reported-by: Jorge Peixoto de Morais Neto <jorge13515@gmail.com>
<http://permalink.gmane.org/gmane.emacs.orgmode/106610>

lisp/org.el

@@ -24098,17 +24098,18 @@ This command will look at the current kill and check if is a single
 subtree, or a series of subtrees[1].  If it passes the test, and if the
 cursor is at the beginning of a line or after the stars of a currently
 empty headline, then the yank is handled specially.  How exactly depends
-on the value of the following variables, both set by default.
+on the value of the following variables.
 
 `org-yank-folded-subtrees'
-    When set, the subtree(s) will be folded after insertion, but only
-    if doing so would now swallow text after the yanked text.
+    By default, this variable is non-nil, which results in subtree(s)
+    being folded after insertion, but only if doing so would now
+    swallow text after the yanked text.
 
 `org-yank-adjusted-subtrees'
-    When set, the subtree will be promoted or demoted in order to
-    fit into the local outline tree structure, which means that the
-    level will be adjusted so that it becomes the smaller one of the
-    two *visible* surrounding headings.
+    When non-nil (the default value is nil), the subtree will be
+    promoted or demoted in order to fit into the local outline tree
+    structure, which means that the level will be adjusted so that it
+    becomes the smaller one of the two *visible* surrounding headings.
 
 Any prefix to this command will cause `yank' to be called directly with
 no special treatment.  In particular, a simple \\[universal-argument] prefix \