refactor: minor refactors & commentary revision

* lisp/doom-cli.el:
  - reference backport source commit.
  - doom-cli--restart: a type check is all we need here. This is a
    programmer error, not a user error.
* lisp/doom-editor.el (recentf): mention recentf-show-abbreviated (added in
  emacs-mirror/emacs@32906819ad)
* lisp/doom-keybinds.el (doom-init-leader-keys-h): move to
  doom-after-init-hook, in case the user customizes leader variables in
  a previous hook (like emacs-startup-hook or after-init-hook).
* lisp/doom-start.el: use eval-when! to compile out the section on
  non-macOS systems (when Doom gets around to compiling its core files,
  later).
* modules/config/literate/autoload.el (+literate-config-file): use
  file-name-concat instead of string concat. This relaxes the
  requirement that doom-user-dir end in a /; a requirement I intend to
  fully phase out.
* modules/lang/emacs-lisp/autoload.el (+emacs-lisp-non-package): remove
  empty map! macro in flycheck-emacs-lisp-check-form. The macro already
  no-ops at compile-time/in noninteractive sessions since b480ed51a3.
* modules/ui/hl-todo/config.el (hl-todo-keyword-faces): revise
  commentary for default hl-todo keywords.

Ref: emacs-mirror/emacs@32906819ad
Ref: b480ed51a3

lisp/doom-start.el

@@ -120,7 +120,7 @@
 ;;   focus when it is started, among other things, so enable the menu-bar for
 ;;   GUI frames, but keep it disabled in terminal frames because there it
 ;;   activates an ugly, in-frame menu bar.
-(when IS-MAC
+(eval-when! IS-MAC
   (add-hook! '(window-setup-hook after-make-frame-functions)
     (defun doom-restore-menu-bar-in-gui-frames-h (&optional frame)
       (when-let (frame (or frame (selected-frame)))