Clarify the current OSXIsGUIApplication() status in comments.

Mention that it doesn't work yet in 3.0 branch.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75146 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775