Describe how the debugging macros really work since 2.9.1, i.e. explain wxDEBUG_LEVEL and NDEBUG instead of __WXDEBUG__. Closes #14435. git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71887 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775