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/ARCHITECTURE.md

@@ -291,10 +291,11 @@ therefore marks a backup that did not finish.
 - With `--verify`, `verifyRestoredFiles` reads each restored regular file and
   checks its chunk hashes.
 
-Given a snapshot ID, the restore does not read the local index. Given a remote
-key, it reads the local index only to check the destination marker. It does not
-read the manifest. It finds each file's chunks and blobs through the downloaded
-database.
+Given a snapshot ID, the restore looks it up in the local index. When it is not
+there, the restore checks the destination marker and lists the snapshot's
+directory. Given a remote key, it reads the local index only to check the
+destination marker. It does not read the manifest. It finds each file's chunks
+and blobs through the downloaded database.
 
 ### Checks on the downloaded database