Fix other sentences that carry faults the review named

A search of the four documents, the help text and config.example.yml
for the review's faults found these.

- prune help, and snapshot purge and snapshot list in README.md: a
  snapshot is dropped or reported when its manifest is gone, not its
  metadata or directory.
- README.md, version: the date printed is the commit date.
- README.md, Configuration: names the commands that start without a
  config that loads.
- README.md, snapshot purge: "each such snapshot" named nothing.
- README.md and docs/DATAMODEL.md: a backup can upload unchanged
  content again, so "uploads only what changed" was false.
- docs/REPOSTRUCTURE.md: db.zst.age holds more than one snapshot's
  rows.
- ARCHITECTURE.md: "database records" read as a noun.

Model: fable-5-1

internal/cli/prune.go

@@ -20,7 +20,7 @@ from the backup destination store.
 Needs permission to list, read and delete objects on the destination
 store. It does not need the age private key.
 
-Local cleanup drops snapshots whose metadata is gone from the
+Local cleanup drops snapshots whose manifest is gone from the
 destination store, incomplete snapshots, and any files, chunks, or
 blobs no longer referenced by a completed snapshot. Remote cleanup
 scans every snapshot manifest in the destination store, builds the