Document wxOSX_FILEDIALOG_ALWAYS_SHOW_TYPES system option

Make this option slightly more discoverable.

See #12429.

interface/wx/filedlg.h

@@ -108,6 +108,18 @@ const char wxFileSelectorDefaultWildcardStr[];
     descriptive test; "BMP files (*.bmp)|*.bmp" is displayed as "*.bmp", and both
     "BMP files (*.bmp)|*.bmp|GIF files (*.gif)|*.gif" and "Image files|*.bmp;*.gif"
     are errors.
+    On Mac OS X in the open file dialog the filter choice box is not shown by default.
+    Instead all given wildcards are appplied at the same time: So in the above
+    example all bmp, gif and png files are displayed. To enforce the
+    display of the filter choice set the corresponding wxSystemOptions before calling
+    the file open dialog:
+    @code
+         wxSystemOptions::SetOption(wxOSX_FILEDIALOG_ALWAYS_SHOW_TYPES, 1)
+    @endcode
+    But in contrast to Windows and Unix, where the file type choice filters only
+    the selected files, on Mac OS X even in this case the dialog shows all files
+    matching all file types. The files which does not match the currently selected
+    file type are greyed out and are not selectable.
 
     @beginStyleTable
     @style{wxFD_DEFAULT_STYLE}