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

modules/lib/file-type.nix

@@ -19,7 +19,7 @@ in
         enable = mkOption {
           type = types.bool;
           default = true;
-          description = lib.mdDoc ''
+          description = ''
             Whether this file should be generated. This option allows specific
             files to be disabled.
           '';
@@ -32,7 +32,7 @@ in
             in
               removePrefix (homeDirectory + "/") absPath;
           defaultText = literalExpression "name";
-          description = lib.mdDoc ''
+          description = ''
             Path to target file relative to ${basePathDesc}.
           '';
         };
@@ -40,7 +40,7 @@ in
         text = mkOption {
           default = null;
           type = types.nullOr types.lines;
-          description = lib.mdDoc ''
+          description = ''
             Text of the file. If this option is null then
             [](#opt-${opt}._name_.source)
             must be set.
@@ -49,7 +49,7 @@ in
 
         source = mkOption {
           type = types.path;
-          description = lib.mdDoc ''
+          description = ''
             Path of the source file or directory. If
             [](#opt-${opt}._name_.text)
             is non-null then this option will automatically point to a file
@@ -60,7 +60,7 @@ in
         executable = mkOption {
           type = types.nullOr types.bool;
           default = null;
-          description = lib.mdDoc ''
+          description = ''
             Set the execute bit. If `null`, defaults to the mode
             of the {var}`source` file or to `false`
             for files created through the {var}`text` option.
@@ -70,7 +70,7 @@ in
         recursive = mkOption {
           type = types.bool;
           default = false;
-          description = lib.mdDoc ''
+          description = ''
             If the file source is a directory, then this option
             determines whether the directory should be recursively
             linked to the target location. This option has no effect
@@ -87,7 +87,7 @@ in
         onChange = mkOption {
           type = types.lines;
           default = "";
-          description = lib.mdDoc ''
+          description = ''
             Shell commands to run when file has changed between
             generations. The script will be run
             *after* the new files have been linked
@@ -102,7 +102,7 @@ in
           type = types.bool;
           default = false;
           visible = false;
-          description = lib.mdDoc ''
+          description = ''
             Whether the target path should be unconditionally replaced
             by the managed file source. Warning, this will silently
             delete the target regardless of whether it is a file or