Document the type and name of the surrounding job in JobLauncherTestUtils#launchStep

[fmbenhassine]

JobLauncherTestUtils#launchStep creates a surrounding single-step job of type SimpleJob which is named TestJob. This does not seem to be documented and should be clarified in the docs.

Reference: https://stackoverflow.com/questions/65569056/how-is-job-name-getting-assigned-in-spring-batch-junit