Fix three sentences that kept faults the review named

The review of the rewrite named these faults in other files. The same
sentences were still in two documents.

- docs/REPOSTRUCTURE.md, "The marker file": "index records" read as a
  noun. It now says the next backup would not upload data that the
  local index says is stored.
- docs/REPOSTRUCTURE.md, "Chunk boundaries use no secret": a chunk is
  not always between 2.5 MB and 40 MB. Only a file's last chunk can be
  smaller than a quarter of chunk_size.
- ARCHITECTURE.md, "Snapshot metadata": the export does not delete
  everything outside the snapshot. It now says what is deleted, and
  that local_meta and schema_migrations are kept.

Model: fable-5-1

docs/ARCHITECTURE.md

@@ -257,7 +257,12 @@ data again.
 itself:
 
 - It copies the index into a temporary directory with `VACUUM INTO`.
-- It deletes from the copy everything that does not belong to this snapshot.
+- It deletes the rows of every other snapshot from the copy. It also deletes the
+  files and blobs that this snapshot does not use. It then deletes each chunk
+  that no remaining file and no remaining blob uses.
+- It keeps the `local_meta` and `schema_migrations` tables as they are. The
+  uploaded database therefore names the destination, and lists the migrations
+  that have run.
 - It runs `VACUUM`, so that no deleted row is left in the file.
 - It compresses the copy with zstd and encrypts it with age.
 - It builds the manifest from the copy.