Bug 21926: Enhance OAI-PMH:archiveID system preference description

Bug 21926: Enhance OAI-PMH:archiveID system preference description

Test plan:
1) Apply the patch
2) Go to Administration -> System preferences -> Web services, section
OAI-PMH
3) See description on OAI-PMH:archiveID and confirm it does make sense
and have the link to implementation guidelines

Signed-off-by: Michal Denar <black23@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>