pool: Fix help text and multi-value support of PNFSID filter

Ticket: http://rt.dcache.org/Ticket/Display.html?id=7994
Require-book: no
Require-notes: yes
Target: trunk
Request: 2.7
Request: 2.6
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/5998/