change vhost to ssl_vhost, add header_name explanation in comments.

letsencrypt-apache/letsencrypt_apache/configurator.py

@@ -772,9 +772,12 @@ class ApacheConfigurator(augeas_configurator.AugeasConfigurator):
 
         Checks to see if virtualhost already contains a header_name header
 
-        :param vhost: vhost to check
+        :param ssl_vhost: vhost to check
         :type vhost: :class:`~letsencrypt_apache.obj.VirtualHost`
 
+        :param header_name: a header name, e.g: Strict-Transport-Security
+        :type str
+
         :returns: boolean
         :rtype: (bool)