docs: address review — make S3/B2/SFTP/local guidance actionable

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.

web/templates/pages/host_repo.html

@@ -82,12 +82,14 @@
         <div class="text-[12px] text-ok mb-3 mono">✓ saved</div>
       {{end}}
       <p class="text-[12.5px] text-ink-mid leading-[1.6] mb-4 max-w-[640px]">
-        Only needed for rest-server repos that distinguish an append-only
-        user (everyday backups) from a delete-capable user (prune). Note
-        that <strong>forget</strong> always runs with the everyday repo
-        credentials, so those must have delete authority. For S3 / B2 /
-        SFTP / local, leave this blank — but note that <strong>prune is
-        skipped entirely when this is unset</strong>, on any backend.
+        Required for prune. On rest-server repos this is the
+        delete-capable user, as distinct from the append-only user used
+        for everyday backups. Note that <strong>forget</strong> always
+        runs with the everyday repo credentials, so those must have
+        delete authority. For S3 / B2 / SFTP / local, enter the same
+        delete-capable repository credentials here if you want prune
+        enabled. <strong>Prune is skipped when admin credentials are
+        unset</strong>, on any backend.
       </p>
       <div class="grid grid-cols-2 gap-4">
         <div>