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