Format the rewritten documents with prettier

Re-wraps README.md, docs/ARCHITECTURE.md, docs/DATAMODEL.md,
docs/REPOSTRUCTURE.md and the roadmap in docs/TODO.md with `make fmt`,
which now runs prettier on Markdown. Only line wrapping changes; the
words are those that passed review at 4da102c.

Model: fable-5-1

docs/ARCHITECTURE.md

@@ -128,8 +128,8 @@ These commands do not build the application:
 - `EnsureStorageBinding` checks that the local index belongs to the configured
   destination.
 - `markDestination` checks the marker file and then writes it.
-- `PruneDatabase` removes unfinished snapshots and unused records from the
-  local index.
+- `PruneDatabase` removes unfinished snapshots and unused records from the local
+  index.
 - `createNamedSnapshot` runs once for each snapshot name.
 - With `--prune`, `runPostBackupPrune` removes old snapshots and unused data.
 
@@ -142,8 +142,7 @@ these cases:
   not find.
 
 `PruneDatabase` must succeed before anything is scanned. An unfinished snapshot
-left in the index would make the backup skip files that were never fully
-stored.
+left in the index would make the backup skip files that were never fully stored.
 
 ### One snapshot
 
@@ -299,8 +298,8 @@ database.
 
 ### Checks on the downloaded database
 
-Decryption shows that a database is readable. It does not show that the
-database is honest. The restore therefore checks it:
+Decryption shows that a database is readable. It does not show that the database
+is honest. The restore therefore checks it:
 
 - `database.OpenReadOnly` opens the file read-only. It refuses a file that has a
   trigger, a view or a virtual table, or that lacks an expected table.
@@ -317,8 +316,8 @@ database is honest. The restore therefore checks it:
 
 - Downloaded blobs are kept decrypted in a cache directory.
 - A file is restored once every blob it needs is in the cache.
-- When no file is ready, the restore downloads the blobs for the file that
-  needs the fewest.
+- When no file is ready, the restore downloads the blobs for the file that needs
+  the fewest.
 - `restoreSweeper` deletes a cached blob once no remaining file needs it.
 
 A regular file is created with mode `0600` and gets its stored mode after its
@@ -351,8 +350,8 @@ the private key:
 
 - `CleanupLocalSnapshots` drops from the local index each snapshot whose
   manifest is gone from the destination.
-- `PruneDatabase` removes unfinished snapshots and unused records from the
-  local index.
+- `PruneDatabase` removes unfinished snapshots and unused records from the local
+  index.
 - `PruneBlobs` deletes unused data at the destination.
 
 `PruneBlobs` reads every manifest at the destination. It then deletes two kinds