20261015 实现RRDP失步检测与TA证书择优

docs/usage.md

@@ -36,6 +36,14 @@ TAL public-key hash. Refresh failures may use that cache under
 `allow-valid-cache` and produce `partial`; use `require-fresh` to reject the
 fallback.
 
+Automatic TA refresh compares usable fetched and cached certificates using the
+[TA tiebreaker rules](configuration.md#5-anchor-fields). A preferred cached
+certificate after a successful fetch keeps the run successful. RRDP also checks
+the previous notification's delta hashes (RFC 9697); a changed hash within the
+same session emits `rrdp_session_desynchronized` and triggers a fresh snapshot,
+including when the notification serial has not advanced. This check uses stored
+hashes and does not fetch old deltas.
+
 ## Multiple anchors and constraints
 
 Use one named `[[anchors]]` table per RIR. There is no positional TAL/TA pairing: