Clarify Emacs version requirement

And stop forcing pgtk for the example packages.

README.md

@@ -56,6 +56,8 @@ Next, you have two options:
     #   doomDir = ./path/to/doom/config;
     # instead.
     doomLocalDir = "~/.local/share/nix-doom";
+    # Emacs package to build on (any Emacs >= 29 should work).
+    emacs = pkgs.emacs29;
   })
   ```
 
@@ -98,7 +100,8 @@ welcome, as are (within reason) changes necessary to support use without flakes.
 
 - `emacs`: Emacs package to use. Defaults to `pkgs.emacs`. Must be at least
   Emacs 29. Use this to select different Emacs variants like
-  `pkgs.emacs29-pgtk`.
+  `pkgs.emacs29-pgtk`. Required in Nixpkgs < 24.05, where `pkgs.emacs` is Emacs
+  28.
 
 - `doomSource`: Doom source tree. Defaults to a flake input: overriding that
   input is probably easier than passing this.

flake.nix

@@ -35,7 +35,8 @@
         let
           common = {
             doomSource = doomemacs;
-            emacs = pkgs.emacs29-pgtk;
+            # TODO: drop after NixOS 24.05 release.
+            emacs = pkgs.emacs29;
             doomLocalDir = "~/.local/share/nix-doom-unstraightened";
           };
           pkgsWithEmacsOverlay = pkgs.extend emacs-overlay.overlays.package;