optimistic-transaction: remove reference to being "default" (pingcap#…

…3146) (pingcap#3202)

* cherry pick pingcap#3146 to release-3.0

Signed-off-by: ti-srebot <ti-srebot@pingcap.com>

* Update optimistic-transaction.md

Co-authored-by: Null not nil <67764674+nullnotnil@users.noreply.github.com>

optimistic-transaction.md

@@ -9,7 +9,7 @@ aliases: ['/docs/v3.0/optimistic-transaction/','/docs/v3.0/reference/transaction
 
 This document introduces the principles of TiDB's optimistic transaction model. This document assumes that you have a basic understanding of [TiDB architecture](/architecture.md), [Percolator](https://ai.google/research/pubs/pub36726), and the [ACID](/glossary.md#acid) properties of transactions.
 
-In TiDB's optimistic transaction model, the two-phase commit begins right after the client executes the `COMMIT` statement. Therefore, the write-write conflict can be observed before the transactions are actually committed.
+In TiDB's optimistic transaction model, write-write conflicts are detected only at the two-phase transactional commit.
 
 > **Note:**
 >