doc: remove optional builtins prefixes from prelude functions

Remove optional builtins prefixes from prelude functions by running:

    builtins=(
      abort
      baseNameOf
      break
      derivation
      derivationStrict
      dirOf
      false
      fetchGit
      fetchMercurial
      fetchTarball
      fetchTree
      fromTOML
      import
      isNull
      map
      null
      placeholder
      removeAttrs
      scopedImport
      throw
      toString
      true
    )

    fd --type file . doc --exec-batch sed --in-place --regexp-extended "
      s/\<builtins\.($(
        printf '%s\n' "${builtins[@]}" |
          paste --delimiter '|' --serial -
      ))\>/\1/g
    "

    nix fmt

doc/languages-frameworks/dhall.section.md

@@ -89,7 +89,7 @@ buildDhallPackage {
 # ./example.nix
 
 let
-  nixpkgs = builtins.fetchTarball {
+  nixpkgs = fetchTarball {
     url = "https://github.com/NixOS/nixpkgs/archive/94b2848559b12a8ed1fe433084686b2a81123c99.tar.gz";
     hash = "sha256-B4Q3c6IvTLg3Q92qYa8y+i4uTaphtFdjp+Ir3QQjdN0=";
   };