Add documentation for the keep-alive flag
Add documentation for the keep-alive flag

This commit updates the documentation of the task:executor element to
reference the keep-alive flag.

Issue: SPR-12407

spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework,spring-projects/spring-framework