ticdc: Update ticdc mysql sink description (pingcap#4981)

* Update ticdc-overview.md

* refine the language

* Apply suggestions from code review

Co-authored-by: TomShawn <41534398+TomShawn@users.noreply.github.com>

ticdc/ticdc-overview.md

@@ -58,9 +58,10 @@ Currently, the TiCDC sink component supports replicating data to the following d
 
 - MySQL sink
 
-    - TiCDC does not split in-table transactions. This is to **ensure** the transactional consistency within a single table. However, TiCDC does **not ensure** consistency with the transactional order in the upstream table. If the primary keys between transactions have intersections, TiCDC still **ensures** consistency with the transactional order in the upstream table.
-    - TiCDC splits cross-table transactions in the unit of tables. TiCDC does **not ensure** that cross-table transactions are always consistent.
-    - TiCDC **ensures** that the order of single-row updates are consistent with that in the upstream.
+    - TiCDC does not split single-table transactions and **ensures** the atomicity of single-table transactions.
+    - TiCDC does **not ensure** that the execution order of downstream transactions is the same as that of upstream transactions.
+    - TiCDC splits cross-table transactions in the unit of table and does **not ensure** the atomicity of cross-table transactions.
+    - TiCDC **ensures** that the order of single-row updates is consistent with that in the upstream.
 
 - Kafka sink