Document that wxCANCEL_DEFAULT doesn't work in wxOSX/Cocoa.

See #16268.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76646 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775