Add an option to not use intersphinx references as a fallback

[adamtheturtle]

That intersphinx references are used as fallbacks if a reference is not available in a project recently caused us a problem.

We have a site (latest) which is the latest build of master.

We use intersphinx as follows, so we can link to the latest documentation in some sections:

intersphinx_mapping = {
    'latest': ('http://doc-dev.clusterhq.com/', None),
}

Make sure to follow the :ref:`latest documentation <latest:release-process>` when doing a release.

Our docs included a label (.. foo:) so this reference was available on latest.
This label was removed on a branch, along with one of the two references to this label.
In this case we would expect that building the branch would fail, as there is a reference to a label which doesn't exist.
Unexpectedly, the branch built successfully, because the label was found in latest as a fall back when it was not found locally. The branch was merged, and then later, when latest changed (because master was built again) builds stopped working because there was a reference to a non-existent label.

It would be good to have an option to not fall back, maybe something like nitpicky.

[nijel]

The initial PR adding this is #8981