infra: turn on warnings as errors for docs builds (#1240)

Author: aaronmarkham

doc/Makefile

@@ -2,8 +2,8 @@
 #
 
 # You can set these variables from the command line.
-SPHINXOPTS    =
-SPHINXBUILD   = python -msphinx
+SPHINXOPTS    = -W
+SPHINXBUILD   = python -msphinx 
 SPHINXPROJ    = sagemaker
 SOURCEDIR     = .
 BUILDDIR      = _build
@@ -17,4 +17,4 @@ help:
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
 %: Makefile
-	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)