Doc warnings as errors

This commit sets warning-is-error = 1 under the build_sphinx
section of setup.cfg, now the standard across many OS services,
to flag sphinx warnings as errors.

Change-Id: I30a5425d7b6a221121350044ff3d197464b43cdb
diff --git a/setup.cfg b/setup.cfg
index a8732dc..eae0d8b 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -27,6 +27,7 @@
 source-dir = doc/source
 build-dir = doc/build
 all_files = 1
+warning-is-error = 1
 
 [upload_sphinx]
 upload-dir = doc/build/html