OAIRepository: OAI-PMH handler URL documented

* Officially documents the OAI-PMH handler URL.
  (closes #1027) (PR #2152)

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>
Merge branch 'maint-1.1'

* maint-1.1:
  OAIRepository: OAI-PMH handler URL documented

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>