doc/stdenv.xml document substitution env variables

The filtering of environment variables that start with an uppercase
letter is documented in the manual.

pkgs/build-support/substitute/substitute-all.nix

@@ -2,6 +2,7 @@
 
 args:
 
+# see the substituteAll in the nixpkgs documentation for usage and constaints
 stdenv.mkDerivation ({
   name = if args ? name then args.name else baseNameOf (toString args.src);
   builder = ./substitute-all.sh;