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

README.md

@@ -53,7 +53,7 @@ It was written to meet these requirements together:
 - **Blob**: one object at the destination. It holds many chunks, compressed and
   then encrypted.
 - **Local index**: a SQLite database on the backed-up machine. It records what
-  is already stored, so the next backup uploads only what changed.
+  is already stored, so the next backup does not upload it again.
 - **Manifest**: the list of blobs one snapshot uses. It is stored at the
   destination without encryption.
 - **Remote key**: a hash of the snapshot ID. It names the snapshot's directory
@@ -405,8 +405,8 @@ the local index. Its timestamp is the time its backup finished.
 
 The command reports these below the table:
 
-- Snapshots in the local index that are missing at the destination. Run
-  `vaultik prune` to drop those records.
+- Snapshots that are in the local index but have no manifest at the destination.
+  Run `vaultik prune` to drop them from the local index.
 - Remote-only snapshots whose manifest cannot be read. They are left out of the
   table.
 - Remote-only snapshots beyond the first 1000. They are left out of the table.
@@ -493,13 +493,14 @@ Flags:
 The command acts only on snapshots in the local index. `--keep-latest` keeps the
 newest snapshot of each name, not the newest overall.
 
-The command removes each such snapshot from the local index. It also deletes the
-snapshot's directory under `metadata/` at the destination. Its blobs stay. Run
-`vaultik prune` afterwards to delete the blobs that no snapshot uses any more.
+The command removes from the local index each snapshot that the flags select. It
+also deletes the snapshot's directory under `metadata/` at the destination. Its
+blobs stay. Run `vaultik prune` afterwards to delete the blobs that no snapshot
+uses any more.
 
-The command first drops local records of snapshots that are no longer at the
-destination. It fails and removes nothing when the destination cannot be listed
-or has no marker file.
+The command first drops from the local index each snapshot whose manifest is
+gone from the destination. It fails and removes nothing when the destination
+cannot be listed or has no marker file.
 
 ### `snapshot remove <snapshot-id>`
 
@@ -732,16 +733,18 @@ vaultik completion fish > ~/.config/fish/completions/vaultik.fish
 
 ### `version`
 
-Prints the version, the commit, the build date and the platform. It also says
-when the binary is a development build, not a release.
+Prints the version, the commit, the commit date and the platform. It labels the
+commit date `build date`. It also says when the binary is a development build,
+not a release.
 
 ## Configuration
 
 The config file is YAML. `vaultik config init` writes one to start from, and
 [`config.example.yml`](config.example.yml) is a longer example.
 
-A command refuses to start when the config does not load. The config loads when
-all of these hold:
+A command refuses to start when the config does not load. The `config` commands,
+`completion` and `version` are the exceptions. The config loads when all of
+these hold:
 
 - Every entry in `age_recipients` is an age X25519 public key.
 - `snapshots` has at least one name, and every name has at least one path.