Add additional detail to explain alert "Backup job is too busy"

A question was raised about what the alert "Backup job is too busy" actually means. A customer was unsure what threshold to set. Having a backup job that is too busy sounds like something they should know about, but they weren't sure what it means.

I provided an answer based on consultation with A&I and https://jira.mongodb.org/browse/CLOUDP-7383.

Can we incorporate some of the detail in CLOUDP-7383 into the docs:
https://docs.opsmanager.mongodb.com/current/reference/alerts/#Backup-job-is-busy-for...

Thanks
Annette