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/DATAMODEL.md

@@ -299,6 +299,7 @@ A backup works on the tables in this order:
 - It walks each path. It compares each regular file and symlink with its row in
   `files`.
 - It links each unchanged file to the snapshot in `snapshot_files`.
+- It writes a `files` row and a `snapshot_files` row for each directory.
 - It records each mount point in `snapshot_mount_points`.
 - It reads each changed regular file and stores it. See below. A changed symlink
   is recorded without being read.
@@ -362,9 +363,9 @@ still be in the `-wal` file beside it.
 
 ### Restore
 
-A restore reads no file, chunk or blob rows from the local index. Given a remote
-key, it reads `snapshots` and `local_meta` there to check the destination
-marker.
+A restore reads no file, chunk or blob rows from the local index. It reads
+`snapshots` and `local_meta` there, to look up a snapshot ID and to check the
+destination marker.
 
 - It downloads and decrypts `db.zst.age` into a temporary directory.
 - It opens that file read-only. It applies no migration to it.