lib-symbol-visibility: Improve documentation. Reported by Vivien Kraus <vivien@planete-kraus.eu> in <https://lists.gnu.org/archive/html/bug-gnulib/2022-06/msg00093.html>. * doc/lib-symbol-visibility.texi: List the platforms. Extend the LIBFOO_DLL_EXPORTED to work also with mingw and also with --disable-shared.