Increase Sphinx strictness.

Warnings are now considered build errors.

We've been shipping documentation with sections missing
because we didn't take warnings seriously.

Bonus change: build in parallel, save 82 centiseconds per build!
(html target on my laptop, ymmv)

doc/sphinx/Makefile.am

@@ -3,7 +3,7 @@
 
 # You can set these variables from the command line.
 SPHINXOPTS    =
-SPHINXBUILD   = sphinx-build
+SPHINXBUILD   = sphinx-build -j4 -W -N -n
 PAPER         = a4
 BUILDDIR      = build