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/installation/changing-config.xml

@@ -10,7 +10,7 @@ contains the current configuration of your machine.  Whenever you’ve
 changed something to that file, you should do
 
 <screen>
-$ nixos-rebuild switch</screen>
+# nixos-rebuild switch</screen>
 
 to build the new configuration, make it the default configuration for
 booting, and try to realise the configuration in the running system
@@ -23,7 +23,7 @@ either run them from a root shell or by prefixing them with
 <para>You can also do
 
 <screen>
-$ nixos-rebuild test</screen>
+# nixos-rebuild test</screen>
 
 to build the configuration and switch the running system to it, but
 without making it the boot default.  So if (say) the configuration
@@ -33,7 +33,7 @@ configuration.</para>
 <para>There is also
 
 <screen>
-$ nixos-rebuild boot</screen>
+# nixos-rebuild boot</screen>
 
 to build the configuration and make it the boot default, but not
 switch to it now (so it will only take effect after the next
@@ -44,7 +44,7 @@ of the GRUB 2 boot screen by giving it a different <emphasis>profile
 name</emphasis>, e.g.
 
 <screen>
-$ nixos-rebuild switch -p test </screen>
+# nixos-rebuild switch -p test </screen>
 
 which causes the new configuration (and previous ones created using
 <literal>-p test</literal>) to show up in the GRUB submenu “NixOS -