arti-client: add comment to `TorClient::reconfigure`, and use `Reconfigure::cannot_change` for `DirMgrConfig::cache_trust`

The doc comment for TorClient::reconfigure comes from #1721 (comment 3126770).

I also noticed that reconfiguring DirMgrConfig::cache_trust is ignored but has no Reconfigure::cannot_change, so I added one.

Part of #1721.

Assigning nickm since the change is small, and it's a followup from discussion on that issue.