pkgsStatic: Add warning saying this is a provisional backport

pkgs/top-level/stage.nix

@@ -163,7 +163,7 @@ let
 
     # Fully static packages.
     # Currently uses Musl on Linux (couldn’t get static glibc to work).
-    pkgsStatic = nixpkgsFun ({
+    pkgsStatic = lib.warn "This a provisional API backported from master for convenience." (nixpkgsFun ({
       crossOverlays = [ (import ./static.nix) ];
     } // lib.optionalAttrs stdenv.hostPlatform.isLinux {
       crossSystem = {
@@ -176,7 +176,7 @@ let
             or lib.systems.parse.abis.musl;
         };
       };
-    });
+    }));
   };
 
   # The complete chain of package set builders, applied from top to bottom.