improve RepositoryUpdate doc

  - make clear that there is only one RepositoryUpdate section

Change-Id: Ia2c2ea8d97f706c6318747745a327e4c07c9bbeb
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Niels Weber <niels.weber@digia.com>