doc/en/Makefile: allow passing in REGENDOC_FILES

This allows for:

> make -C doc/en regen REGENDOC_FILES=example/parametrize.rst

Author: blueyed

doc/en/Makefile

@@ -39,8 +39,9 @@ help:
 clean:
 	-rm -rf $(BUILDDIR)/*
 
+regen: REGENDOC_FILES:=*.rst */*.rst
 regen:
-	PYTHONDONTWRITEBYTECODE=1 PYTEST_ADDOPTS=-pno:hypothesis COLUMNS=76 regendoc --update *.rst */*.rst ${REGENDOC_ARGS}
+	PYTHONDONTWRITEBYTECODE=1 PYTEST_ADDOPTS=-pno:hypothesis COLUMNS=76 regendoc --update ${REGENDOC_FILES} ${REGENDOC_ARGS}
 
 html:
 	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html