sslutil: document the Apple OpenSSL cert trick

This is sort of documented in _plainapplypython()'s docstring. But
it helps to be explicit in security code.

mercurial/sslutil.py

@@ -442,6 +442,9 b' def _defaultcacerts(ui):'
     except ImportError:
         pass
 
+    # Apple's OpenSSL has patches that allow a specially constructed certificate
+    # to load the system CA store. If we're running on Apple Python, use this
+    # trick.
     if _plainapplepython():
         dummycert = os.path.join(os.path.dirname(__file__), 'dummycert.pem')
         if os.path.exists(dummycert):