Add attention description for service file change

Add attention description for service file change

When integrate some service in guest container, that need to drop tty
dependency.  AGL compositor has tty dependency at default.
This dependency drop using drop-in config in meta-agl-demo layer.

This patch add attention description for service file change to
avoid break.

Bug-AGL: SPEC-4888

Change-Id: Iaeae73b6d81c87f4d61d5829f49e8ab01f84a3f6
Signed-off-by: Naoto Yamaguchi <naoto.yamaguchi@aisin.co.jp>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/29174
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>