Add documentation for custom model actions

- Add plugin development guide for registering custom actions
- Update admin permissions docs to mention custom actions UI
- Add docstrings to ModelAction and register_model_actions

docs/administration/permissions.md

@@ -20,7 +20,9 @@ There are four core actions that can be permitted for each type of object within
 * **Change** - Modify an existing object
 * **Delete** - Delete an existing object
 
-In addition to these, permissions can also grant custom actions that may be required by a specific model or plugin. For example, the `run` permission for scripts allows a user to execute custom scripts. These can be specified when granting a permission in the "additional actions" field.
+In addition to these, permissions can also grant custom actions that may be required by a specific model or plugin. For example, the `sync` action for data sources allows a user to synchronize data from a remote source, and the `render_config` action for devices and virtual machines allows rendering configuration templates.
+
+Some models have registered custom actions that appear as checkboxes when creating or editing a permission. These are grouped by model under "Custom actions" in the permission form. Additional custom actions (such as those not yet registered or for backwards compatibility) can be entered manually in the "Additional actions" field.
 
 !!! note
     Internally, all actions granted by a permission (both built-in and custom) are stored as strings in an array field named `actions`.