makeShell: improve update-nix-bitcoin

- Don't overwrite `nix-bitcoin-release.nix` on errors
- Show a message to indicate whether `nix-bitcoin-release.nix` was
  updated
- Don't start a shell when called noninteractively

Also, update `usage.md` and reformat `shell.nix`.

examples/shell.nix

@@ -4,5 +4,5 @@ in
   import "${nix-bitcoin}/helper/makeShell.nix" {
     configDir = ./.;
     # Set this to modify your shell
-    # extraShellInitCmds = (pkgs: ''<my bash code>'');
+    # extraShellInitCmds = pkgs: ''<my bash code>'';
   }