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/administration/container-networking.xml

@@ -13,7 +13,7 @@ create</literal>, it gets it own private IPv4 address in the range
 address as follows:
 
 <screen>
-$ nixos-container show-ip foo
+# nixos-container show-ip foo
 10.233.4.2
 
 $ ping -c1 10.233.4.2
@@ -47,4 +47,4 @@ where <literal>eth0</literal> should be replaced with the desired
 external interface. Note that <literal>ve-+</literal> is a wildcard
 that matches all container interfaces.</para>
 
-</section>
+</section>