better document function.nix

Signed-off-by: Kiara Grouwstra <kiara@procolix.eu>

deployment/function.nix

@@ -1,5 +1,38 @@
 /**
-  Modular function type
+  Modular function type.
+
+  Compared to plain nix functions, adds input type-checks
+  at the cost of longer stack traces.
+
+  Usage:
+
+  ```nix
+  { lib, ... }:
+  {
+    options = {
+      my-function = lib.mkOption {
+        description = "My type-safe function invocation.";
+        type = lib.types.submodule PATH/TO/function.nix;
+        readOnly = true;
+        default = {
+          input-type = lib.types.int;
+          output-type = lib.types.int;
+          implementation = x: x + x;
+        };
+      };
+    };
+    config = {
+      my-function.apply "1"
+    };
+  }
+  ```
+
+  A sample stack trace using this ends up like:
+
+  - `INVOKER.apply.<function body>``
+  - `function.nix`
+  - `INVOKER.wrapper.<function body>.output`
+  - `INVOKER.implementation.<function body>`
 */
 { config, lib, ... }:
 let