Fixed #27086 -- Doc'd fix for test suite hang on macOS.

docs/internals/contributing/writing-code/unit-tests.txt

@@ -380,6 +380,20 @@ Ensure you have the latest point release of a :ref:`supported Python version
 <faq-python-version-support>`, since there are often bugs in earlier versions
 that may cause the test suite to fail or hang.
 
+On **macOS** (High Sierra and newer versions), you might see this message
+logged, after which the tests hang::
+
+    objc[42074]: +[__NSPlaceholderDate initialize] may have been in progress in
+    another thread when fork() was called.
+
+To avoid this set a ``OBJC_DISABLE_INITIALIZE_FORK_SAFETY`` environment
+variable, for example::
+
+    $ OBJC_DISABLE_INITIALIZE_FORK_SAFETY=YES ./runtests.py
+
+Or add ``export OBJC_DISABLE_INITIALIZE_FORK_SAFETY=YES`` to your shell's
+startup file (e.g. ``~/.profile``).
+
 Many test failures with ``UnicodeEncodeError``
 ----------------------------------------------