Found it, see

On Tuesday, December 1, 2015 at 5:00:22 PM UTC-6, Josh Warner wrote:
Per @blink1037's Sphinx guide at my best guess is it has to do with the references in though none actually appear to be malformatted on a cursory glance.

On Tuesday, December 1, 2015 at 4:18:16 PM UTC-6, Jonathan Helmus wrote:
On 12/01/2015 03:26 PM, Stéfan van der Walt wrote:
> It looks like the docs build is breaking unrelated to #1794:
> Any ideas?
Sphinx 1.3.2 was released a few days back, could something in this
release have broken the doc build?

    - Jonathan Helmus