Fix nine review findings in the documents and help text

- snapshot verify and snapshot restore also list the destination for a
  snapshot ID that is not in the local index: README, REPOSTRUCTURE,
  ARCHITECTURE, DATAMODEL and the two commands' help.
- prune on a separate machine deletes only unused data and does not
  remove old snapshots.
- The --quiet usage string names everything the flag leaves out.
- A warning written as a log record stays on stderr.
- config set on a config that does not load refuses three things.
- No test is held back by a build tag.
- A plain docker build . adds no -dirty.
- The backup's order of work records directories.
- TODO.md says what parallel downloads would speed up and names
  restorePlan and restoreSweeper.

Model: fable-5-1

docs/REPOSTRUCTURE.md

@@ -59,7 +59,8 @@ not exist, commands treat the destination as not present:
     - `prune`
     - `snapshot purge`
     - `remote nuke`
-    - `snapshot verify` and `snapshot restore`, when given a remote key
+    - `snapshot verify` and `snapshot restore`, when given a remote key, or a
+      snapshot ID that is not in the local index
 - `snapshot list` shows the local index alone.
 - `snapshot remove` deletes nothing at the destination.
 
@@ -334,12 +335,14 @@ An upload there looks up the object it replaces and deletes it. It stops when
 the look-up is refused.
 
 `snapshot verify` and `snapshot restore` list only when they are given a remote
-key. `snapshot remove --local-only` does not reach the destination.
+key, or a snapshot ID that is not in the local index.
+`snapshot remove --local-only` does not reach the destination.
 `snapshot remove --dry-run` deletes nothing.
 
 `snapshot create --prune` runs a prune on the backed-up machine. That machine
 then holds credentials that can delete its own backups. To keep it to write-only
-credentials, run `prune` from a separate, trusted machine.
+credentials, run `prune` from a separate, trusted machine. `prune` there deletes
+only data that no snapshot uses. It does not remove old snapshots.
 
 ### New recipients cannot read old data