treewide: remove now-redundant lib.mdDoc calls

These (and the `*MD` functions apart from `literalMD`) are now no-ops
in nixpkgs and serve no purpose other than to add additional noise and
potentially mislead people into thinking unmarked DocBook documentation
will still be accepted.

Note that if backporting changes including documentation to 23.05,
the `mdDoc` calls will need to be re-added.

To reproduce this commit, run:

    $ NIX_PATH=nixpkgs=flake:nixpkgs/e7e69199f0372364a6106a1e735f68604f4c5a25 \
      nix shell nixpkgs#coreutils \
      -c find . -name '*.nix' \
      -exec nix run -- github:emilazy/nix-doc-munge/98dadf1f77351c2ba5dcb709a2a171d655f15099 \
      --strip {} +
    $ ./format

nixos/common.nix

@@ -43,20 +43,20 @@ let
 
 in {
   options.home-manager = {
-    useUserPackages = mkEnableOption (mdDoc ''
+    useUserPackages = mkEnableOption ''
       installation of user packages through the
-      {option}`users.users.<name>.packages` option'');
+      {option}`users.users.<name>.packages` option'';
 
-    useGlobalPkgs = mkEnableOption (mdDoc ''
+    useGlobalPkgs = mkEnableOption ''
       using the system configuration's `pkgs`
       argument in Home Manager. This disables the Home Manager
-      options {option}`nixpkgs.*`'');
+      options {option}`nixpkgs.*`'';
 
     backupFileExtension = mkOption {
       type = types.nullOr types.str;
       default = null;
       example = "backup";
-      description = mdDoc ''
+      description = ''
         On activation move existing files by appending the given
         file extension rather than exiting with an error.
       '';
@@ -66,7 +66,7 @@ in {
       type = types.attrs;
       default = { };
       example = literalExpression "{ inherit emacs-overlay; }";
-      description = mdDoc ''
+      description = ''
         Extra `specialArgs` passed to Home Manager. This
         option can be used to pass additional arguments to all modules.
       '';
@@ -76,19 +76,19 @@ in {
       type = with types; listOf raw;
       default = [ ];
       example = literalExpression "[ { home.packages = [ nixpkgs-fmt ]; } ]";
-      description = mdDoc ''
+      description = ''
         Extra modules added to all users.
       '';
     };
 
-    verbose = mkEnableOption (lib.mdDoc "verbose output on activation");
+    verbose = mkEnableOption "verbose output on activation";
 
     users = mkOption {
       type = types.attrsOf hmModule;
       default = { };
       # Prevent the entire submodule being included in the documentation.
       visible = "shallow";
-      description = mdDoc ''
+      description = ''
         Per-user Home Manager configuration.
       '';
     };