treewide: convert all option docs to Markdown

This process was automated by [my fork of `nix-doc-munge`]. All
conversions were automatically checked to produce the same DocBook
result when converted back, modulo minor typographical/formatting
differences on the acceptable-to-desirable spectrum.

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 \
    {} +
  $ ./format

[my fork of `nix-doc-munge`]: https://github.com/emilazy/nix-doc-munge/tree/home-manager

modules/programs/gitui.nix

@@ -10,14 +10,14 @@ in {
   meta.maintainers = [ hm.maintainers.mifom ];
 
   options.programs.gitui = {
-    enable =
-      mkEnableOption "gitui, blazing fast terminal-ui for git written in rust";
+    enable = mkEnableOption
+      (lib.mdDoc "gitui, blazing fast terminal-ui for git written in rust");
 
     package = mkOption {
       type = types.package;
       default = pkgs.gitui;
       defaultText = "pkgs.gitui";
-      description = "The package to use.";
+      description = lib.mdDoc "The package to use.";
     };
 
     keyConfig = mkOption {
@@ -28,9 +28,9 @@ in {
         quit: Some(( code: Char('q'), modifiers: ( bits: 0,),)),
         exit_popup: Some(( code: Esc, modifiers: ( bits: 0,),)),
       '';
-      description = ''
+      description = lib.mdDoc ''
         Key config in Ron file format. This is written to
-        <filename>$XDG_CONFIG_HOME/gitui/key_config.ron</filename>.
+        {file}`$XDG_CONFIG_HOME/gitui/key_config.ron`.
       '';
     };
 
@@ -61,9 +61,9 @@ in {
           branch_fg: LightYellow,
         )
       '';
-      description = ''
+      description = lib.mdDoc ''
         Theme in Ron file format. This is written to
-        <filename>$XDG_CONFIG_HOME/gitui/theme.ron</filename>.
+        {file}`$XDG_CONFIG_HOME/gitui/theme.ron`.
       '';
     };
   };