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

ARCHITECTURE.md

@@ -299,7 +299,7 @@ database is honest. The restore therefore checks it:
   the remote key that was asked for.
 - `containedRestorePath` refuses a stored path that would leave the target
   directory. It also refuses to write through a symlink below the target.
-- A blob is not decompressed beyond the size the database records for it.
+- A blob is not decompressed beyond the size that the database gives for it.
 - Each downloaded blob is hashed. The hash must match the blob's name.
 
 ### Order of work