Add on_safe_mode trigger to docs (#3857)

components/safe_mode.rst

@@ -5,8 +5,6 @@ Safe Mode
     :description: Instructions for setting up ESPHome's Safe Mode to help recover from repeated boot failures.
     :image: system-update.svg
 
-.. _config-safe_mode:
-
 Sometimes hardware and/or software doesn't behave as expected. ESPHome supports a "safe mode" to help recover from
 repeated boot failures/reboot loops. After a specified number (the default is ten) of boot failures, the safe mode may
 be invoked; in this mode, all components are disabled except serial logging, network (Wi-Fi or Ethernet) and the OTA
@@ -22,6 +20,7 @@ for ``num_attempts`` times (see below).
     # Example configuration entry
     safe_mode:
 
+.. _safe_mode-configuration_variables:
 
 Configuration variables:
 ------------------------
@@ -32,6 +31,16 @@ Configuration variables:
    Defaults to ``10``.
 -  **reboot_timeout** (*Optional*, :ref:`config-time`): The amount of time to wait before rebooting when in safe mode.
    Defaults to ``5min``.
+- **on_safe_mode** (*Optional*, :ref:`Automation <automation>`): An action to be performed once when safe mode is invoked.
+
+.. warning::
+
+    The ``on_safe_mode`` :ref:`automation <automation>` is intended for use by recovery actions **only**.
+    
+    As mentioned above, in safe mode, all components are disabled except serial logging, network (Wi-Fi or Ethernet)
+    and OTA component(s).
+    
+    **All other components (for example, displays and sensors) are disabled and cannot be used.**
 
 See Also
 --------

guides/automations.rst

@@ -349,6 +349,7 @@ All Triggers
   :ref:`ota.on_state_change <ota-on_state_change>`
 - :ref:`display.on_page_change <display-on_page_change-trigger>`
 - :ref:`cover.on_open <cover-on_open_trigger>` / :ref:`cover.on_closed <cover-on_closed_trigger>`
+- :ref:`safe_mode.on_safe_mode <safe_mode-configuration_variables>`
 - :ref:`wifi.on_connect / wifi.on_disconnect <wifi-on_connect_disconnect>`
 
 All Actions