Make sphinx-build warnings fatal

In order to prevent documentation building issues from being glossed
over, pass the -W command line switch to all sphinx-build invocations.
This causes the latter to return with a non-zero exit code whenever any
Sphinx warnings are triggered.

Makefile.docs

@@ -10,6 +10,7 @@ AM_V_SPHINX_0 = @echo "  SPHINX   $@";
 SPHINXBUILDDIR = $(builddir)/_build
 
 common_SPHINXOPTS =			\
+	-W				\
 	-c $(srcdir)			\
 	-a				\
 	$(SPHINX_V)