htmlxref.cnf: New file to let texi2html point at the correct Calc HTML manual

doc/htmlxref.cnf

@@ -0,0 +1,2 @@
+calc		mono	http://www.gnu.org/software/emacs/manual/html_mono/calc.html
+calc		node	http://www.gnu.org/software/emacs/manual/html_node/calc/