Document that a bootstrap executor for the context is auto-configured

Document that `applicationTaskExecutor` as a bootstrap executor for the
context if no `bootstrapExecutor` bean is defined.

See gh-45232

Signed-off-by: Dmytro Nosan <[email protected]>

Author: nosan

Diff for: spring-boot-project/spring-boot-docs/src/docs/antora/modules/reference/pages/features/task-execution-and-scheduling.adoc

@@ -13,6 +13,7 @@ The auto-configured javadoc:org.springframework.core.task.AsyncTaskExecutor[] is
 - Support for blocking execution in Spring WebFlux.
 - Utilized for inbound and outbound message channels in Spring WebSocket.
 - Acts as a bootstrap executor for JPA, based on the bootstrap mode of JPA repositories.
+- Used as `bootstrapExecutor` for parallel bean initialization during startup, unless a `bootstrapExecutor` bean is defined.
 
 While this approach works in most scenarios, Spring Boot allows you to override the auto-configured javadoc:org.springframework.core.task.AsyncTaskExecutor[].
 By default, when a custom javadoc:java.util.concurrent.Executor[] bean is registered, the auto-configured javadoc:org.springframework.core.task.AsyncTaskExecutor[] backs off, and the custom javadoc:java.util.concurrent.Executor[] is used for regular task execution (via javadoc:org.springframework.scheduling.annotation.EnableAsync[format=annotation]).
@@ -31,6 +32,12 @@ include-code::application/MyTaskExecutorConfiguration[]
 The `applicationTaskExecutor` bean will also be used for regular task execution if there is no javadoc:org.springframework.context.annotation.Primary[format=annotation] bean or a bean named `taskExecutor` of type javadoc:java.util.concurrent.Executor[] or javadoc:org.springframework.scheduling.annotation.AsyncConfigurer[] present in the application context.
 ====
 
+[NOTE]
+====
+The `applicationTaskExecutor` bean will be used for background bootstrapping if a bean named `bootstrapExecutor` is not defined.
+Spring Boot registers an alias for the `applicationTaskExecutor` bean with the name `bootstrapExecutor` if a `bootstrapExecutor` bean is not already defined and `applicationTaskExecutor` is present in the context.
+====
+
 [WARNING]
 ====
 If neither the auto-configured `AsyncTaskExecutor` nor the `applicationTaskExecutor` bean is defined, the application defaults to a bean named `taskExecutor` for regular task execution (javadoc:org.springframework.scheduling.annotation.EnableAsync[format=annotation]), following Spring Framework's behavior.
@@ -80,6 +87,7 @@ These integrations include:
 - Spring WebFlux's blocking execution support.
 - Utilized for inbound and outbound message channels in Spring WebSocket.
 - Acts as a bootstrap executor for JPA, based on the bootstrap mode of JPA repositories.
+- Used as the `bootstrapExecutor` for parallel bean initialization during startup, unless a `bootstrapExecutor` bean is explicitly provided.
 
 [TIP]
 ====