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/REPOSTRUCTURE.md

@@ -60,8 +60,8 @@ not present:
 
 The check is skipped when the look-up fails for any other reason. S3 refuses
 the look-up to credentials that can only write. A machine with such credentials
-therefore does not notice an emptied destination. Its next backup would leave
-out the data its local index records as stored. After a destination is emptied,
+therefore does not notice an emptied destination. Its next backup would not
+upload data that its local index says is stored. After a destination is emptied,
 run `vaultik database delete` on such a machine before it backs up again.
 
 `remote nuke` deletes the marker last.
@@ -275,9 +275,10 @@ Back up a directory that outsiders can write to as a snapshot of its own.
 
 ### Chunk boundaries use no secret
 
-The FastCDC parameters are fixed and public. With the default `chunk_size` of
-10 MB, a chunk is between 2.5 MB and 40 MB. A file of 2.5 MB or less is one
-chunk.
+The FastCDC parameters are fixed and public. `chunk_size` sets the average chunk
+size. No chunk is larger than four times this size. Only a file's last chunk can
+be smaller than a quarter of it. With the default of 10 MB, those limits are 40
+MB and 2.5 MB. A file of 2.5 MB or less is one chunk.
 
 At the default `blob_size_limit` of 10 GB, a blob holds hundreds of chunks. The
 size of the blob then does not show the size of any one chunk. A limit close to