docs: document unstable packages with flakes

Add a flake-based example for using a secondary nixpkgs-unstable input from Home Manager modules, matching the existing channel-based FAQ guidance.

Show how standalone configurations pass the package set with extraSpecialArgs, and explain the equivalent home-manager.extraSpecialArgs path for NixOS and nix-darwin module users.

Document that separate package sets do not automatically inherit the primary package set's nixpkgs.config or overlays, and show how to pass those values when needed.

docs/manual/faq/unstable.md

@@ -3,22 +3,20 @@
 If you are using a stable version of Nixpkgs but would like to install
 some particular packages from Nixpkgs unstable -- or some other channel
 -- then you can import the unstable Nixpkgs and refer to its packages
-within your configuration. Something like
+within your configuration.
+
+With channels, something like
 
 ``` nix
 { pkgs, config, ... }:
 
 let
-
   pkgsUnstable = import <nixpkgs-unstable> {};
-
 in
-
 {
   home.packages = [
     pkgsUnstable.foo
   ];
-
   # …
 }
 ```
@@ -32,5 +30,82 @@ $ nix-channel --add https://nixos.org/channels/nixpkgs-unstable nixpkgs-unstable
 $ nix-channel --update
 ```
 
-Note, the package will not be affected by any package overrides,
-overlays, etc.
+With flakes, add another Nixpkgs input and pass its packages to your
+Home Manager modules. For example, a standalone Home Manager flake can
+define the following. Replace 25.11 with the release branch your
+configuration follows.
+
+``` nix
+{
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.11";
+    nixpkgs-unstable.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    home-manager.url = "github:nix-community/home-manager/release-25.11";
+  };
+
+  outputs =
+    { nixpkgs, nixpkgs-unstable, home-manager, ... }:
+    let
+      # Replace this with the system of your Home Manager configuration.
+      system = "x86_64-linux";
+      pkgs = nixpkgs.legacyPackages.${system};
+      pkgsUnstable = nixpkgs-unstable.legacyPackages.${system};
+    in
+    {
+      homeConfigurations.jdoe = home-manager.lib.homeManagerConfiguration {
+        inherit pkgs;
+        extraSpecialArgs = { inherit pkgsUnstable; };
+        modules = [ ./home.nix ];
+      };
+    };
+}
+```
+
+and then use the extra argument in `home.nix`:
+
+``` nix
+{ pkgsUnstable, ... }:
+
+{
+  home.packages = [
+    pkgsUnstable.foo
+  ];
+}
+```
+
+When Home Manager is used as a NixOS or nix-darwin module, pass the
+extra package set with `home-manager.extraSpecialArgs` in the system
+configuration:
+
+``` nix
+outputs =
+  { nixpkgs, nixpkgs-unstable, home-manager, ... }:
+  let
+    # Replace this with the system of your NixOS or nix-darwin configuration.
+    system = "x86_64-linux";
+  in
+  {
+    nixosConfigurations.hostname = nixpkgs.lib.nixosSystem {
+      inherit system;
+      modules = [
+        home-manager.nixosModules.home-manager
+        ({ config, ... }: {
+          home-manager.extraSpecialArgs = {
+            pkgsUnstable = import nixpkgs-unstable {
+              inherit system;
+              config = config.nixpkgs.config;
+              overlays = config.nixpkgs.overlays;
+            };
+
+            # If you use a stock Nixpkgs configuration, you can use:
+            # pkgsUnstable = nixpkgs-unstable.legacyPackages.${system};
+          };
+        })
+      ];
+    };
+  };
+```
+
+The nix-darwin setup is the same pattern with
+`darwin.lib.darwinSystem` and
+`home-manager.darwinModules.home-manager`.