Add reference for xfail message

Author: XuehaiPan

tests/test_multiple_interpreters.py

@@ -408,7 +408,11 @@ def test_import_in_subinterpreter_before_main():
 @pytest.mark.skipif(
     sys.platform.startswith("emscripten"), reason="Requires loadable modules"
 )
-@pytest.mark.xfail(env.MUSLLINUX, reason="Flaky on musllinux", strict=False)
+@pytest.mark.xfail(
+    env.MUSLLINUX,
+    reason="Flaky on musllinux, see also: https://github.com/pybind/pybind11/pull/5972#discussion_r2755283335",
+    strict=False,
+)
 @pytest.mark.skipif(not CONCURRENT_INTERPRETERS_SUPPORT, reason="Requires 3.14.0b3+")
 def test_import_in_subinterpreter_concurrently():
     """Tests that importing a module in multiple subinterpreters concurrently works correctly"""