The previous wording told operators to leave the slot blank and then
warned that doing so disables prune, which is contradictory. Prune is
gated on the admin slot for every backend, on both paths:
scheduled (maintenance_dispatch.go:52) skips silently, manual
(repo_ops.go:39) returns 400 admin_creds_required.
Also reworded the opening line, which still said 'only needed for
rest-server repos' while the new text asks other backends to fill it in.
Forget does not use admin credentials. Only JobPrune builds a runner
against the admin slot (cmd/agent/main.go:622-645); JobForget runs on
the everyday runner, as the comment at main.go:517-524 states.
Also corrects the claim that leaving this blank lets everyday creds
handle prune: maintenance_dispatch.go:52-56 skips prune entirely with
'prune skipped — no admin creds' on any backend, with no fallback.
Refs #34
Steve Cliff