Document secondary flag for device automations (#1839)

* Document secondary flag for device automations

* Update device_automation_index.md

docs/device_automation_index.md

@@ -8,3 +8,42 @@ Device Automations provide users with a device-centric layer on top of the core
 Integrations can hook into this system by exposing functions to generate the pre-defined triggers, conditions, actions and having functions that can listen for the triggers, check the condition and execute the action.
 
 Device automations are not exposing extra functionality but are a way for users to not have to learn new concepts. Device automations are using events, state and service helpers under the hood.
+
+### Secondary device automations
+
+Some devices may expose a lot of device automation. To not overwhelm the user, a device automation can be marked as secondary. A device automation which is marked as secondary will still be shown to the user, but may be shown after other device automations or may require the user to select a "show more" option or similar.
+
+If the device automation references an entity via an `entity_id` key, the secondary flag will automatically be set to `True` if the referenced entity is hidden or if the referenced entity's entity category is not `None`. The example below shows how to mark a device automation as secondary.
+
+```python
+from homeassistant.const import (
+    CONF_DEVICE_ID,
+    CONF_DOMAIN,
+    CONF_PLATFORM,
+    CONF_TYPE,
+)
+from homeassistant.helpers import device_registry as dr
+
+async def async_get_triggers(hass, device_id):
+    """Return a list of triggers."""
+
+    device_registry = dr.async_get(hass)
+    device = device_registry.async_get(device_id)
+
+    triggers = []
+
+    # Determine which triggers are supported by this device_id ...
+
+    triggers.append({
+        # Required fields of TRIGGER_BASE_SCHEMA
+        CONF_PLATFORM: "device",
+        CONF_DOMAIN: "mydomain",
+        CONF_DEVICE_ID: device_id,
+        # Required fields of TRIGGER_SCHEMA
+        CONF_TYPE: "less_important_trigger",
+        # Mark the trigger as secondary
+        "metadata": {"secondary": True},
+    })
+
+    return triggers
+```