Add documentation about the transform function

docs/plugins/index.rst

@@ -19,17 +19,17 @@ To do so you can use `entry points <https://setuptools.readthedocs.io/en/latest/
 
 Using entrypoints in your ``setup.py``::
 
-    setup(
-        # ...
-        entry_points={
-          "PATH": "NAME = path.to:Plugin",
-        }
-    )
+  setup(
+    # ...
+    entry_points={
+      "PATH": "NAME = path.to:Plugin",
+    }
+  )
 
 In your pyproject.toml if using ``poetry``::
 
-    [tool.poetry.plugins."PATH"]
-    "NAME" = "path.to:Plugin"
+  [tool.poetry.plugins."PATH"]
+  "NAME" = "path.to:Plugin"
 
 Where PATH is:
 
@@ -40,18 +40,18 @@ Where PATH is:
 
 Where NAME is the way you want to refer to it later on and ``path.to:Plugin`` the import path. For instance::
 
-    [tool.poetry.plugins."nornir.plugins.inventory"]
-    "inventory-name" = "path.to:InventoryPlugin"
+  [tool.poetry.plugins."nornir.plugins.inventory"]
+  "inventory-name" = "path.to:InventoryPlugin"
 
 
 To do it programmatically import the correct plugin register and use the ``register`` method. For instance::
 
-    from nornir.core.plugins.inventory import InventoryPluginRegister
+  from nornir.core.plugins.inventory import InventoryPluginRegister
 
-    from path.to import InventoryPlugin
+  from path.to import InventoryPlugin
 
 
-    InventoryPluginRegister.register("inventory-name", InventoryPlugin)
+  InventoryPluginRegister.register("inventory-name", InventoryPlugin)
 
 Connections
 -----------
@@ -75,6 +75,8 @@ Transform functions
 
 A transform function is a plugin that manipulates the inventory independently from the inventory plugin used. Useful to extend data using the environment, a secret store or similar.
 
+During inventory initialization, the transform function will be called in a for loop for each host. The transform function takes a host object as the first parameter and additional keyword arguments as specified in the `config.inventory.transform_function_options` dictionary.
+
 Runners
 _______