Add reference to PR #4268 (PyPy Windows)

Author: rwgk

include/pybind11/detail/common.h

@@ -264,7 +264,9 @@
 
 // See description of PR #4246:
 #if !defined(NDEBUG) && !defined(PY_ASSERT_GIL_HELD_INCREF_DECREF)                                \
-    && !(defined(PYPY_VERSION) && defined(_MSC_VER)) /* Tests hang indefinitely at startup. */    \
+    && !(defined(PYPY_VERSION)                                                                    \
+         && defined(_MSC_VER)) /* PyPy Windows: pytest hangs indefinitely at the end of the       \
+                                  process (see PR #4268) */                                       \
     && !defined(PYBIND11_ASSERT_GIL_HELD_INCREF_DECREF)
 #    define PYBIND11_ASSERT_GIL_HELD_INCREF_DECREF
 #endif