[PATCH 6 of 8] sslutil: document the Apple Python cert trick

[Gregory Szorc]

# HG changeset patch
# User Gregory Szorc <gregory.szorc at gmail.com>
# Date 1467427907 25200
#      Fri Jul 01 19:51:47 2016 -0700
# Node ID 17d67895b215c5c977d2b446611abb219160a6f9
# Parent  101e040f8b014fe938803c68b502e7b72b4726f8
sslutil: document the Apple Python cert trick

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

diff --git a/mercurial/sslutil.py b/mercurial/sslutil.py
--- a/mercurial/sslutil.py
+++ b/mercurial/sslutil.py
@@ -438,16 +438,19 @@ def _defaultcacerts(ui):
     with demandimport.deactivated():
         try:
             import certifi
             ui.debug('using ca certificates from certifi\n')
             return certifi.where()
         except Exception:
             pass
 
+    # Apple's Python 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):
             return dummycert
 
     return None
 
 def validatesocket(sock):