doc: allow sphinx to create pure text file docs

doc/Makefile

@@ -11,10 +11,11 @@ PAPEROPT_a4     = -D latex_paper_size=a4
 PAPEROPT_letter = -D latex_paper_size=letter
 ALLSPHINXOPTS   = -d _build/doctrees $(PAPEROPT_$(PAPER)) $(SPHINXOPTS) .
 
-.PHONY: help clean html dirhtml pickle json htmlhelp qthelp latex changes linkcheck doctest
+.PHONY: help clean text html dirhtml pickle json htmlhelp qthelp latex changes linkcheck doctest
 
 help:
 	@echo "Please use \`make <target>' where <target> is one of"
+	@echo "  text      to make text files"
 	@echo "  html      to make standalone HTML files"
 	@echo "  dirhtml   to make HTML files named index.html in directories"
 	@echo "  pickle    to make pickle files"
@@ -30,6 +31,11 @@ help:
 clean:
 	-rm -rf _build/*
 
+text:
+	$(SPHINXBUILD) -b text $(ALLSPHINXOPTS) _build/text
+	@echo
+	@echo "Build finished. The text files are in _build/text."
+
 html:
 	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) _build/html
 	@echo