Bug 30822: Clarify that BatchCommitItems is a private function

Bug 30822: Clarify that BatchCommitItems is a private function

BatchCommitItems is only being used within this module and isn't
mentioned in EXPORT_OK. This patch simply renames it to
_batchCommitItems to take the _ standard for private functions and also
adds a little hint to the POD of the function to clarify that the caller
must trigger a re-index.

JK: Amended patch to rename also the function in t/db_dependent/ImportBatch.t
    and fix typo "commiting" => "commiting"

Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>