Updates per code review

Author: zuohaocheng

docs/api.rst

@@ -559,9 +559,9 @@ Extended handlers
 
   .. data:: client_certfile
 
-    The path of the certificate to check the client certificate against. 
-    When provided, only allowing clients with a valid certificate to connect 
-    to the server (default ``None``).
+    The path to a file which contains a certificate to be used to identify
+    the client. If specified, only clients with a valid certificate are able
+    to connect to the server (default ``None``).
 
      .. versionadded:: 1.5.3    
 

pyftpdlib/handlers.py

@@ -3419,7 +3419,6 @@ class TLS_FTPHandler(SSLConnection, FTPHandler):
         certfile = None
         keyfile = None
         ssl_protocol = SSL.SSLv23_METHOD
-        # client certificate configurable attributes
         client_certfile = None
         # - SSLv2 is easily broken and is considered harmful and dangerous
         # - SSLv3 has several problems and is now dangerous