Fix wxDataViewCtrl::GetItemRect() signature in wxOSX and document it better.

The second parameter of this method should be optional, so make it so in wxOSX.

Also document what this method does exactly and the platforms under which it's
implemented.

Closes #13914.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70496 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775