Merge pull request #5926 from tk0miya/update_docs

Fix doc: html_baseurl was added in 1.8.0

doc/usage/configuration.rst

@@ -864,6 +864,8 @@ that use Sphinx's HTMLWriter class.
    The URL which points to the root of the HTML documentation.  It is used to
    indicate the location of document like ``canonical_url``.
 
+   .. versionadded:: 1.8
+
 .. confval:: html_context
 
    A dictionary of values to pass into the template engine's context for all