mingw: Document mode wbx workaround

src/test/streams_tests.cpp

@@ -30,8 +30,7 @@ BOOST_AUTO_TEST_CASE(xor_file)
     }
     {
 #ifdef __MINGW64__
-        // Our usage of mingw-w64 and the msvcrt runtime does not support
-        // the x modifier for the _wfopen().
+        // Temporary workaround for https://github.com/bitcoin/bitcoin/issues/30210
         const char* mode = "wb";
 #else
         const char* mode = "wbx";