Bug 1420891 - Docs: Enable strict & 'nit-picky' mode when building

So that the build fails if there are any warnings:
http://www.sphinx-doc.org/en/stable/invocation.html#invocation-of-sphinx-build

docs/Makefile

@@ -2,7 +2,7 @@
 #
 
 # You can set these variables from the command line.
-SPHINXOPTS    =
+SPHINXOPTS    = -n -W
 SPHINXBUILD   = sphinx-build
 PAPER         =
 BUILDDIR      = _build