Since 0.18, Element.traverse() returns an iterator instead of
intermediate object.  As a result, the return value is always considered
as truthy value.  And it becomes fragile when the caller modifies the
doctree on the loop.

Fix #8870: Use title node for the caption of toctree

Since docutils-0.17, the caption node should not use for the non-figure
nodes.  Therefore, this replaces it by the title node.

To describe the purpose more accurately, the `master_doc` is now renamed
to `root_doc`.  The old name is still available.  But it is recommeneded
to use new one from now on.