Mention incompatible wxGridBagSizer::AddGrowable{Col,Row}() change.

It now has no effect if called before adding the columns/rows to the sizer.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75111 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775