docs: clarify --del alias, early keep-set caps, ACK wait, --no-delete conflict

Usage help now gives --delete-during a complete description with --del on its
own line, and notes that timing flags imply --delete while timing+--no-delete
is rejected regardless of argument order. RSYNC_COMPAT.md documents: the
receiver's MAX_MANIFEST_ENTRIES/MAX_MANIFEST_BYTES caps now abort an early-mode
run before any data (previously only the deletion step failed), the extended
early-delete ACK deadline, and the order-independent flag-conflict policy.

RSYNC_COMPAT.md

@@ -129,6 +129,30 @@ manifest ack. `--delete-delay` and `--delete-during` are each implemented as
 the closest safe approximation their engine mode allows; the divergences are
 noted in the rows above.
 
+Manifest size: the sender's keep-set collection (streaming or early pre-scan)
+is unbounded, but the receiver rejects any manifest beyond `MAX_MANIFEST_ENTRIES`
+(1 048 576 entries) / `MAX_MANIFEST_BYTES` (16 MB of paths) as a hard protocol
+error. In the commit modes this only means the deletion is refused after the
+data already arrived; in the NEW early modes (`--delete-before`/`--delete-during`)
+the manifest is the first frame, so an oversized keep-set now aborts the whole
+transfer BEFORE any data is sent (previously all data transferred and only the
+deletion step failed). Keep the source tree small enough for the receiver's
+manifest caps when using the early timing.
+
+Early-delete ACK wait: after committing a large deletion (up to
+`MAX_SERVER_DELETE_COUNT` unlinks) the receiver's `STATUS_OK`/`STATUS_ERROR`
+reply can legitimately take much longer than a normal round trip, so the sender
+waits for that single ACK with an extended explicit deadline (1 hour) instead
+of the default 60 s per-message receive window. A receiver that is genuinely
+gone still aborts the wait via connection close/error; the extended bound only
+protects against aborting after the deletion already committed on the receiver.
+
+Flag-conflict policy: unlike rsync's last-one-wins behaviour, every deletion
+timing flag implies `--delete`, and combining a timing flag with `--no-delete`
+(in either argument order) — or more than one timing flag — is rejected as a
+configuration error rather than silently resolved. Note the check is
+order-independent because it runs over the fully parsed config.
+
 ## 8. Metadata Preservation
 
 | Flag | Rsync Description | FastSync Status | Notes |

src/client/usage.c

@@ -28,12 +28,15 @@ void print_usage(void) {
   printf("                      transfer has succeeded)\n");
   printf("  --delete-before     Delete extras before the transfer starts\n");
   printf("                      (implies --delete)\n");
-  printf("  --delete-during     Delete extras once the keep-set is known, before\n");
-  printf("  --del               data is applied (alias --del; implies --delete)\n");
+  printf("  --delete-during     Delete extras once the keep-set manifest is known,\n");
+  printf("                      before the data is applied (implies --delete)\n");
+  printf("  --del               Alias for --delete-during\n");
   printf("  --delete-delay      Delete extras only after a successful transfer\n");
   printf("                      (implies --delete)\n");
-  printf("  --delete-after      Alias of the default --delete timing: delete only\n");
-  printf("                      after the transfer succeeded (implies --delete)\n");
+  printf("  --delete-after      Delete only after the whole transfer succeeded\n");
+  printf("                      (the default --delete timing; implies --delete)\n");
+  printf("  Note: each timing flag implies --delete.  Combining a timing flag with\n");
+  printf("  --no-delete (in either order) is rejected as a config error.\n");
   printf("  --ignore-existing  Skip files that already exist on receiver\n");
   printf("  --delay-updates    Put updated files into place only at the end of transfer\n");
   printf("  --dirs, -d, --old-dirs, --old-d  Transfer the named directory entries without\n");