drm/simple-kms: Fix documentation for drm_simple_encoder_init()

Brings the documentation of drm_simple_encoder_init() in sync with the
function's signature. Also add a paragraph clarifying the management of
the encoder's memory.

v2:
	* document memory management
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Fixes: 63170ac6 ("drm/simple-kms: Add drm_simple_encoder_{init,create}()")
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20200304145312.26458-1-tzimmermann@suse.de

drivers/gpu/drm/drm_simple_kms_helper.c

@@ -38,9 +38,10 @@ static const struct drm_encoder_funcs drm_simple_encoder_funcs_cleanup = {
 };
 
 /**
- * drm_simple_encoder_init - Initialize a preallocated encoder
+ * drm_simple_encoder_init - Initialize a preallocated encoder with
+ *                           basic functionality.
  * @dev: drm device
- * @funcs: callbacks for this encoder
+ * @encoder: the encoder to initialize
  * @encoder_type: user visible type of the encoder
  *
  * Initialises a preallocated encoder that has no further functionality.
@@ -48,6 +49,15 @@ static const struct drm_encoder_funcs drm_simple_encoder_funcs_cleanup = {
  * The encoder will be cleaned up automatically as part of the mode-setting
  * cleanup.
  *
+ * The caller of drm_simple_encoder_init() is responsible for freeing
+ * the encoder's memory after the encoder has been cleaned up. At the
+ * moment this only works reliably if the encoder data structure is
+ * stored in the device structure. Free the encoder's memory as part of
+ * the device release function.
+ *
+ * FIXME: Later improvements to DRM's resource management may allow for
+ *        an automated kfree() of the encoder's memory.
+ *
  * Returns:
  * Zero on success, error code on failure.
  */