Fix the four findings of the second review

README.md: the opening no longer says the private key is needed only to
restore; a backup never needs it, a deep verify does.

ARCHITECTURE.md: the concurrency note says each open database uses one
connection, since the metadata export opens a second one.

docs/REPOSTRUCTURE.md: the size leak is about a secret near the
outsider's data in the same blob, not the same chunk, because all new
chunks of a blob share one zstd stream.

Every path used as link text in the four documents is in backticks.

Model: fable-5-1

docs/REPOSTRUCTURE.md

@@ -149,7 +149,7 @@ The snapshot's database. A restore needs it.
   age.
 
 It is encrypted because it holds file paths, permissions and owners.
-[DATAMODEL.md](DATAMODEL.md) describes its tables.
+[`DATAMODEL.md`](DATAMODEL.md) describes its tables.
 
 ### `manifest.json.zst`
 
@@ -261,15 +261,16 @@ MiB, or 4 MiB at compression levels 1 and 2. A blob is closed at
 `blob_size_limit` and at the end of each configured path. A chunk that is
 already stored is never packed again.
 
-Data that an outsider writes compresses smaller when it matches a secret in the
-same chunk, so the blob gets smaller. The blob's size therefore shows the
+Data that an outsider writes compresses smaller when it matches a secret near it
+in the same blob, so the blob gets smaller. The blob's size therefore shows the
 outsider whether their data matched the secret.
 
 This works only when all of these hold:
 
 - The outsider can write into a backed-up file.
 - The outsider can see the blob sizes at the destination.
-- The outsider's data and the secret land in the same chunk.
+- The outsider's data and the secret are in the same blob, no further apart than
+  the stream looks back.
 - The file keeps changing.
 
 Back up a directory that outsiders can write to as a snapshot of its own.