Update information about wxOSX/Cocoa in the platform overview.

Mention that OS X 10.7 is now required and that Cocoa is preferred to Carbon
now.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77717 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775