org-adapt-indentation: Tweak headline-data addition

* lisp/org.el (org-adapt-indentation): Adjust :safe value for
headline-data, avoid unnecessary quote in :type value, and follow the
convention for quoting symbols in docstrings.

lisp/org.el

@@ -1582,7 +1582,7 @@ When this variable is set to t, Org assumes that you write
 outlines by indenting text in each node to align with the
 headline (after the stars).
 
-When this variable is set to 'headline-data, only adapt the
+When this variable is set to `headline-data', only adapt the
 indentation of the data lines right below the headline, such as
 planning/clock lines and property/logbook drawers.
 
@@ -1608,9 +1608,9 @@ time in Emacs."
   :type '(choice
 	  (const :tag "Adapt indentation for all lines" t)
 	  (const :tag "Adapt indentation for headline data lines"
-		 'headline-data)
+		 headline-data)
 	  (const :tag "Do not adapt indentation at all" nil))
-  :safe #'booleanp)
+  :safe (lambda (x) (memq x '(t nil headline-data))))
 
 (defvaralias 'org-special-ctrl-a 'org-special-ctrl-a/e)