[docs] engine_overview.rst: add length and views parameter to videos template

docs/dev/engines/engine_overview.rst

@@ -339,6 +339,8 @@ type.
    content                   *(not implemented yet)*
    publishedDate             :py:class:`datetime.datetime`, time of publish
    thumbnail                 string, url to a small-preview image
+   length                    :py:class:`datetime.timedelta`, duration of result
+   views                     string, view count in humanized number format
    ========================= =====================================================