sslutil: clarify internal documentation

I ran into this python issue with an incomplete certificate chain on Windows
recently, and this is the clarification that came from that experimenting.  The
comment I left on the bug tracker [1] with a reference to the CPython code [2]
indicates that the original problem I had is a different bug, but happened to
be mentioned under issue20916 on the Python bug tracker.

[1] https://bz.mercurial-scm.org/show_bug.cgi?id=5313#c7
[2] https://hg.python.org/cpython/file/v2.7.12/Modules/_ssl.c#l628

mercurial/sslutil.py

@@ -414,8 +414,10 b' def wrapsocket(sock, keyfile, certfile, '
         # a hint to the user.
         # Only modern ssl module exposes SSLContext.get_ca_certs() so we can
         # only show this warning if modern ssl is available.
-        # The exception handler is here because of
-        # https://bugs.python.org/issue20916.
+        # The exception handler is here to handle bugs around cert attributes:
+        # https://bugs.python.org/issue20916#msg213479.  (See issues5313.)
+        # When the main 20916 bug occurs, 'sslcontext.get_ca_certs()' is a
+        # non-empty list, but the following conditional is otherwise True.
         try:
             if (caloaded and settings['verifymode'] == ssl.CERT_REQUIRED and
                 modernssl and not sslcontext.get_ca_certs()):