doc: Explain our specialArgs

doc/src/SUMMARY.md

@@ -8,6 +8,7 @@
 
 - [Activation](activate.md)
 - [Flake Templates](templates.md)
+- [Module Arguments](specialArgs.md)
 - [Module Outputs](outputs.md)
 - [Autowiring](autowiring.md)
 

doc/src/specialArgs.md

@@ -0,0 +1,31 @@
+# Module Arguments
+
+Each of your NixOS, nix-darwin and home-manager modules implicitly receive a [`specialArgs`](https://nixos.asia/en/nix-modules) called `flake`.
+
+The components of this `flake` attrset are:
+
+| Name | Description |
+| ---- | ----------- |
+| `inputs` | The `inputs` of your flake; `inputs.self` referring to the flake itself |
+| `config` | The flake-parts perSystem `config` |
+
+[Here](https://github.com/srid/nixos-config/blob/a420e5f531172aef753b07a411de8e254207f5c6/modules/darwin/default.nix#L2-L5) is an example of how these can be used:
+
+```nix
+{ flake, pkgs, lib, ... }:
+let
+  inherit (flake) config inputs;
+  inherit (inputs) self;
+in
+{
+  imports = [
+    # Reference a flake input directly from a nix-darwin module
+    inputs.agenix.darwinModules.default
+  ];
+
+  # Reference an arbitrary flake-parts config
+  home-manager.users.${config.me.username} = { };
+}
+```
+
+While the above example uses a nix-darwin module, you can do the same on NixOS or home-manager modules.