Change documentation references from OS X to macOS (#1927)

Since OS X 10.12 it has been named macOS so it makes sense
to reference it in documentation as such, even when it
sometimes refers to older versions which were called (Mac) OS X.

interface/wx/button.h

@@ -32,7 +32,7 @@
 
     A button may have either a single image for all states or different images
     for the following states (different images are not currently supported
-    under OS X where the normal image is used for all states):
+    under macOS where the normal image is used for all states):
     @li @b normal: the default state
     @li @b disabled: bitmap shown when the button is disabled.
     @li @b pressed: bitmap shown when the button is pushed (e.g. while the user
@@ -115,7 +115,7 @@ public:
         @ref page_stockitems "this list", a standard label will be used. In
         other words, if you use a predefined @c wxID_XXX constant, just omit
         the label completely rather than specifying it. In particular, help
-        buttons (the ones with @a id of @c wxID_HELP) under OS X can't
+        buttons (the ones with @a id of @c wxID_HELP) under macOS can't
         display any label at all and while wxButton will detect if the standard
         "Help" label is used and ignore it, using any other label will prevent
         the button from correctly appearing as a help button and so should be