Document unexpected default wxGraphicsContext interpolation quality.

In 3.0, keep wxINTERPOLATION_GOOD as the default value instead of
wxINTERPOLATION_DEFAULT for compatibility, but at least mention this and warn
about the changes in 3.1.

See #14134.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76304 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775