stubgen: docs for llndk-deprecate

This doesn't do anything now, but we are using it
to document an API used to be in the LL-NDK. This
is important so that we know not to remove the
symbol (e.g. if it is only in the LL-NDK and not
in the NDK).

Bug: 317293882
Test: N/A
Change-Id: If67715be5572ade7ce1e7c3bd9bc7a98f3da39f5