docs: Clarify internal vs. external import in template

I'm a Nix noob and just learning, but was a little bit confused about this at first; perhaps this helps to clarify it for others who will come along in the future.

template/default/flake.nix

@@ -9,7 +9,8 @@
   outputs = inputs@{ flake-parts, ... }:
     flake-parts.lib.mkFlake { inherit inputs; } {
       imports = [
-        # To import a flake module
+        # To import an internal flake module: ./other.nix
+        # To import an external flake module:
         #   1. Add foo to inputs
         #   2. Add foo as a parameter to the outputs function
         #   3. Add here: foo.flakeModule