Document that applicationTaskExecutor can also act as bootstrapExecutor if the latter is not explicitly configured

[nosan]

It would be helpful to include this information in task-execution-and-scheduling.adoc.

Related to gh-39791

[deepdharsha]

could you please allow me to contribute?

[nosan]

Thank you for your willingness to contribute, @deepdharsha! That's greatly appreciated!

When I was creating this issue, I already had some materials on a local branch that include information about the bootstrapExecutor, but I forgot to ask for the issue to be assigned to me. Since I originally created this issue, it wouldn't feel fair to ask you or anyone else to update the documentation.

I've included this information in the following PR: #45232. Feel free to review it and suggest any additional details or improvements you think could be useful!

Thanks again!

[philwebb]

Closing in favor of PR #45232. Thanks @nosan!