Added comments to remove Engine.render_to_string in Django 2.0.

Since this is a private API introduced in Django 1.8, no documentation
is required.

django/template/engine.py

@@ -171,16 +171,16 @@ class Engine(object):
             template = Template(template, origin, template_name, engine=self)
         return template
 
+    # This method was originally a function defined in django.template.loader.
+    # It was moved here in Django 1.8 when encapsulating the Django template
+    # engine in this Engine class. It's still called by deprecated code but it
+    # will be removed in Django 2.0. It's superseded by a new render_to_string
+    # function in django.template.loader.
+
     def render_to_string(self, template_name, context=None,
                          context_instance=_context_instance_undefined,
                          dirs=_dirs_undefined,
                          dictionary=_dictionary_undefined):
-        """
-        Loads the given template_name and renders it with the given dictionary as
-        context. The template_name may be a string to load a single template using
-        get_template, or it may be a tuple to use select_template to find one of
-        the templates in the list. Returns a string.
-        """
         if context_instance is _context_instance_undefined:
             context_instance = None
         else:

django/template/loader.py

@@ -100,9 +100,9 @@ def render_to_string(template_name, context=None,
         # Some deprecated arguments were passed - use the legacy code path
         for engine in _engine_list(using):
             try:
-                # This is required for deprecating arguments specific to Django
+                # This is required for deprecating properly arguments specific
-                # templates. Simply return engine.render_to_string(template_name,
+                # to Django templates. Remove Engine.render_to_string() at the
-                # context) in Django 2.0.
+                # same time as this code path in Django 2.0.
                 if isinstance(engine, DjangoTemplates):
                     # Hack -- use the internal Engine instance of DjangoTemplates.
                     return engine.engine.render_to_string(