Document the verification email timeout

https://redmine.teklia.com/issues/7318

Follow-up of backend!2335 (merged): the new job_timeouts.send_verification_email setting must be documented in the configuration docs.

You can also start a draft of the 1.6.2 upgrade notes, similar to the 1.6.1 upgrade notes, to make sysadmins aware of this change. They will not have any action to take if they are fine with the default value for the timeout, but this type of change on async tasks still can have an impact on deployments or later troubleshooting.