synctest: Add sample multiplex file

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
docs: Update documentation with multiplex config docs

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.core.data_dir: Fix docstring

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>