Backport commit dc152c5 from Emacs

Apple changed the spelling of its operating system again, to "macOS",
effective with macOS 10.12 Sierra (2016-09-20).  Change Emacs
documentation and comments to match this.  Stick with older OS
spellings ("OS X", "Mac OS X") when talking about older releases where
the older names are more correct.

Modernize usage of 'macOS' in doc and comments
dc152c54f4e44f5f2040883b03f71ff6aa66c893
Paul Eggert
Sun Nov 6 00:42:03 2016 -0700

lisp/org.el

@@ -2251,7 +2251,7 @@ See `org-file-apps'.")
     ("dvi"    . "xdvi %s")
     ("fig"    . "xfig %s")
     (t . "open %s"))
-  "Default file applications on a MacOS X system.
+  "Default file applications on a macOS system.
 The system \"open\" is known as a default, but we use X11 applications
 for some files for which the OS does not have a good default.
 See `org-file-apps'.")
@@ -2323,7 +2323,7 @@ Possible values for the file identifier are:
                will also open html files inside Emacs, unless you add
                (\"html\" . default) to the list as well.
  `system'      The system command to open files, like `open' on Windows
-               and Mac OS X, and mailcap under GNU/Linux.  This is the command
+               and macOS, and mailcap under GNU/Linux.  This is the command
                that will be selected if you call `org-open-at-point' with a
                double prefix argument (`\\[universal-argument] \
 \\[universal-argument] \\[org-open-at-point]').