1_agl-compositor: Mention agl-shell-app-id for migrating apps

1_agl-compositor: Mention agl-shell-app-id for migrating apps

Besides using the agl-shell-desktop protocol one can use agl-shell-app-id
to migrate surfaces to other local, or remote outputs.

Bug-AGL: SPEC-3820

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: Ifdb5853a9a3eaf9dc041d68596c353548437c64b
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/26136
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>