drm: bridge: document bridge attach/detach imbalance

Since commit 4a878c03 ("drm: bridge: Detach bridge from encoder at
encoder cleanup time"), it is generally no longer correct to detach bridges
from encoders manually. Document that.

Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180806061910.29914-3-peda@axentia.se