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

docs/DATAMODEL.md

@@ -6,7 +6,7 @@ index. This document describes its tables and how each command uses them.
 The local index has two jobs:
 
 - It records which files, chunks and blobs are already stored. The next backup
-  then uploads only what changed.
+  then does not store them again.
 - It is the source of each snapshot's own database. A backup uploads a trimmed,
   encrypted copy of the index with every snapshot.
 

docs/REPOSTRUCTURE.md

@@ -142,8 +142,9 @@ remote key keeps names out of a listing. It does not hide them from a guess.
 
 The snapshot's database. A restore needs it.
 
-- **Contents**: a SQLite database that holds this snapshot only. It lists every
-  file with its metadata, its chunks, and the blob that holds each chunk.
+- **Contents**: a SQLite database. It is a copy of the local index without the
+  rows of other snapshots. It lists every file in the snapshot with its
+  metadata, its chunks, and the blob that holds each chunk.
 - **Format**: the database file, compressed with zstd and then encrypted with
   age.