Manual: Explicitly mark commands that require to be run as root (#15589)

* manual: Mark commands that require root

Mark every command that requires to be run as root by prefixing them
with '#' instead of '$'.

* manual: Add note about commands that require root

nixos/doc/manual/development/testing-installer.xml

@@ -12,16 +12,16 @@ properly:
 
 <screen>
 $ nix-build -A config.system.build.nixos-install
-$ mount -t tmpfs none /mnt
-$ ./result/bin/nixos-install</screen>
+# mount -t tmpfs none /mnt
+# ./result/bin/nixos-install</screen>
 
 To start a login shell in the new NixOS installation in
 <filename>/mnt</filename>:
 
 <screen>
-$ ./result/bin/nixos-install --chroot
+# ./result/bin/nixos-install --chroot
 </screen>
 
 </para>
 
-</chapter>
+</chapter>