avoid markdown which the repl's :doc cannot handle

code blocks, if not surrounded by empty lines, have the language
tags (in these cases, always `nix`) show up in the output of :doc.

for example:

  nix-repl> :doc builtins.parseFlakeRef
  Synopsis: builtins.parseFlakeRef flake-ref

    Parse a flake reference, and return its exploded form.

    For example: nix builtins.parseFlakeRef
    "github:NixOS/nixpkgs/23.05?dir=lib"  evaluates to: nix { dir =
    "lib"; owner = "NixOS"; ref = "23.05"; repo = "nixpkgs"; type =
    "github"; }

is now instead:
  nix-repl> :doc builtins.parseFlakeRef
  Synopsis: builtins.parseFlakeRef flake-ref

    Parse a flake reference, and return its exploded form.

    For example:

      | builtins.parseFlakeRef "github:NixOS/nixpkgs/23.05?dir=lib"

    evaluates to:

      | { dir = "lib"; owner = "NixOS"; ref = "23.05"; repo = "nixpkgs"; type = "github"; }

src/libexpr/flake/flake.cc

@@ -868,10 +868,13 @@ static RegisterPrimOp r3({
       Parse a flake reference, and return its exploded form.
 
       For example:
+
       ```nix
       builtins.parseFlakeRef "github:NixOS/nixpkgs/23.05?dir=lib"
       ```
+
       evaluates to:
+
       ```nix
       { dir = "lib"; owner = "NixOS"; ref = "23.05"; repo = "nixpkgs"; type = "github"; }
       ```
@@ -920,12 +923,15 @@ static RegisterPrimOp r4({
       Convert a flake reference from attribute set format to URL format.
 
       For example:
+
       ```nix
       builtins.flakeRefToString {
         dir = "lib"; owner = "NixOS"; ref = "23.05"; repo = "nixpkgs"; type = "github";
       }
       ```
+
       evaluates to
+
       ```nix
       "github:NixOS/nixpkgs/23.05?dir=lib"
       ```

src/libexpr/primops.cc

@@ -1915,11 +1915,13 @@ static RegisterPrimOp primop_outputOf({
       *`derivation reference`* must be a string that may contain a regular store path to a derivation, or may be a placeholder reference. If the derivation is produced by a derivation, you must explicitly select `drv.outPath`.
       This primop can be chained arbitrarily deeply.
       For instance,
+
       ```nix
       builtins.outputOf
         (builtins.outputOf myDrv "out")
         "out"
       ```
+
       will return a placeholder for the output of the output of `myDrv`.
 
       This primop corresponds to the `^` sigil for derivable paths, e.g. as part of installable syntax on the command line.

src/libexpr/primops/fetchTree.cc

@@ -650,12 +650,14 @@ static RegisterPrimOp primop_fetchGit({
 
         The public keys against which `rev` is verified if `verifyCommit` is enabled.
         Must be given as a list of attribute sets with the following form:
+
         ```nix
         {
           key = "<public key>";
           type = "<key type>"; # optional, default: "ssh-ed25519"
         }
         ```
+
         Requires the [`verified-fetches` experimental feature](@docroot@/contributing/experimental-features.md#xp-feature-verified-fetches).