Document `site_name` attribute of Site - fixes #2099

docs/reference/pages/model_reference.rst

@@ -193,6 +193,14 @@ Database fields
 
         This is the port number that the site responds on.
 
+    .. attribute:: site_name
+
+        (text - optional)
+
+        A human-readable name for the site. This is not used by Wagtail itself, but is suitable for use on the site front-end, such as in ``<title>`` elements.
+
+        For example: ``Rod's World of Birds``
+
     .. attribute:: root_page
 
         (foreign key to :class:`~wagtail.wagtailcore.models.Page`)