Add doc notes about TLS context.

Author: moisesguimaraes

bmemcached/client/mixin.py

@@ -24,6 +24,9 @@ class ClientMixin(object):
     :type unpickler: function
     :param socket_timeout: The timeout applied to memcached connections.
     :type socket_timeout: float
+    :param tls_context: A TLS context in order to connect to TLS enabled
+        memcached servers.
+    :type tls_context: ssl.SSLContext
     """
     def __init__(self, servers=('127.0.0.1:11211',), username=None,
                  password=None, compression=None,