BibIndex: better wording for stemming in admin UI

* Improves the wording of the configuration form for the stemming
  setting, in order to better clarify how the system is affected
  by changing the stemming setting of an index.

Reviewed-by: Tibor Simko <tibor.simko@cern.ch>
Merge branch 'maint-0.99' into maint-1.0

* maint-0.99:
  BibIndex: better wording for stemming in admin UI