docs: add language attribute to program listings

doc/installation.xml

@@ -88,7 +88,7 @@
     <para>
      On NixOS you may need to log out and back in for the channel to become
      available. On non-NixOS you may have to add
-<programlisting>
+<programlisting language="bash">
 export NIX_PATH=$HOME/.nix-defexpr/channels${NIX_PATH:+:}$NIX_PATH
 </programlisting>
      to your shell (see
@@ -113,7 +113,7 @@ export NIX_PATH=$HOME/.nix-defexpr/channels${NIX_PATH:+:}$NIX_PATH
      If you do not plan on having Home Manager manage your shell configuration
      then you must source the
     </para>
-<programlisting>
+<programlisting language="bash">
 $HOME/.nix-profile/etc/profile.d/hm-session-vars.sh
 </programlisting>
     <para>

modules/home-environment.nix

@@ -163,7 +163,7 @@ in
         Note, these variables may be set in any order so no session
         variable may have a runtime dependency on another session
         variable. In particular code like
-        <programlisting>
+        <programlisting language="nix">
           home.sessionVariables = {
             FOO = "Hello";
             BAR = "$FOO World!";
@@ -172,7 +172,7 @@ in
         may not work as expected. If you need to reference another
         session variable, then do so inside Nix instead. The above
         example then becomes
-        <programlisting>
+        <programlisting language="nix">
           home.sessionVariables = {
             FOO = "Hello";
             BAR = "''${config.home.sessionVariables.FOO} World!";

modules/misc/nixpkgs.nix

@@ -80,7 +80,7 @@ in
         inside and outside Home Manager you can put it in a separate
         file and include something like
 
-        <programlisting>
+        <programlisting language="nix">
           nixpkgs.config = import ./nixpkgs-config.nix;
           xdg.configFile."nixpkgs/config.nix".source =
               ./nixpkgs-config.nix;

modules/programs/zsh.nix

@@ -174,7 +174,7 @@ in
         default = true;
         description = ''
           Enable zsh completion. Don't forget to add
-          <programlisting>
+          <programlisting language="nix">
             environment.pathsToLink = [ "/share/zsh" ];
           </programlisting>
           to your system configuration to get completion for system packages (e.g. systemd).