ili9341: document auto_clear_enabled flag (#1568)

Also reference in section about rendering engine.

Co-authored-by: Tim Niemueller <timdn@google.com>

components/display/ili9341.rst

@@ -56,6 +56,8 @@ Configuration variables:
 - **lambda** (*Optional*, :ref:`lambda <config-lambda>`): The lambda to use for rendering the content on the display.
   See :ref:`display-engine` for more information.
 - **update_interval** (*Optional*, :ref:`config-time`): The interval to re-draw the screen. Defaults to ``5s``.
+- **auto_clear_enabled** (*Optional*, boolean): Whether to automatically clear the display in each loop (''true'', default),
+  or to keep the existing display content (must overwrite explicitly, e.g., only on data change).
 - **pages** (*Optional*, list): Show pages instead of a single lambda. See :ref:`display-pages`.
 - **id** (*Optional*, :ref:`config-id`): Manually specify the ID used for code generation.
 

components/display/index.rst

@@ -35,7 +35,9 @@ individually.
 
 So, first a few basics: When setting up a display platform in ESPHome there will be a configuration
 option called ``lambda:`` which will be called every time ESPHome wants to re-render the display.
-In there, you can write code like in any :ref:`lambda <config-lambda>` in ESPHome. Display
+In each cycle, the display is automatically cleared before the lambda is executed. You can disable
+this behavior by setting ``auto_clear_enabled: false``.
+In the lambda, you can write code like in any :ref:`lambda <config-lambda>` in ESPHome. Display
 lambdas are additionally passed a variable called ``it`` which represents the rendering engine object.
 
 .. code-block:: yaml