Update migration transaction docs to reflect EF Core 9 single-transaction behavior

entity-framework/core/managing-schemas/migrations/managing.md

@@ -196,7 +196,10 @@ This can be used to manage any aspect of your database, including:
 * Triggers
 * Views
 
-In most cases, EF Core will automatically wrap each migration in its own transaction when applying migrations. Unfortunately, some migrations operations cannot be performed within a transaction in some databases; for these cases, you may opt out of the transaction by passing `suppressTransaction: true` to `migrationBuilder.Sql`.
+In most cases, EF Core will automatically wrap each migration in its own transaction when applying migrations. Unfortunately, some migration operations cannot be performed within a transaction in some databases; for these cases, you may opt out of the transaction by passing `suppressTransaction: true` to `migrationBuilder.Sql`.
+
+> [!NOTE]
+> In EF Core 9, EF Core spans all pending migrations with a single transaction by default (this was reverted in EF Core 10). See [the breaking change note](xref:core/what-is-new/ef-core-9.0/breaking-changes#migrations-single-transaction) for details.
 
 ## Remove a migration
 

entity-framework/core/what-is-new/ef-core-9.0/breaking-changes.md

@@ -36,6 +36,7 @@ EF Core 9 targets .NET 8. This means that existing applications that target .NET
 | [EF tools no longer support .NET Framework projects](#ef-tools-no-netfx)                                  | Low        |
 | [`EF.Constant()` and `EF.Parameter()` no longer work inside compiled queries](#ef-constant-compiled)      | Low        |
 | [Some `NoTrackingWithIdentityResolution` queries are now prohibited for JSON collections](#no-tracking-json) | Low        |
+| [All pending migrations are applied in a single transaction](#migrations-single-transaction)              | Low        |
 
 ## High-impact changes
 
@@ -450,6 +451,28 @@ var blogs = await context.Blogs
     .ToListAsync();
 ```
 
+<a name="migrations-single-transaction"></a>
+
+### All pending migrations are applied in a single transaction
+
+[Tracking Issue #34439](https://github.com/dotnet/efcore/issues/34439)
+
+#### Old behavior
+
+Previously, EF Core would (by default) wrap each migration in its own transaction when applying migrations.
+
+#### New behavior
+
+Starting with EF Core 9.0, all pending migrations are applied within a single transaction as part of [migration locking](xref:core/managing-schemas/migrations/applying#migration-locking). If any migration fails, the entire transaction is rolled back. This behavior was reverted in EF Core 10.
+
+#### Why
+
+Applying all migrations in a single transaction ensures the database is either fully up-to-date or left unchanged if an error occurs, avoiding intermediate states.
+
+#### Mitigations
+
+If you need per-migration transaction behavior in EF Core 9, upgrade to EF Core 10, which reverts to wrapping each migration in its own transaction.
+
 ## Azure Cosmos DB breaking changes
 
 Extensive work has gone into making the Azure Cosmos DB provider better in 9.0. The changes include a number of high-impact breaking changes; if you are upgrading an existing application, please read the following carefully.