Explicitly note that "Global" Mail Setting options only affect Service Desk

It would be a much better user experience if the Global Mail Settings administration menu explicitly stated that those options only apply to Jira Service Desk. I was only able to discover this after reading Managing the email channel, and it caused quite a few emails to be processed by our development and staging servers.

I propose renaming it to Service Desk Mail Settings, as there is nothing global about it currently, and it is nested with the rest of the standard Jira mail handler settings. At the very least some warning text would spare some confusion for other users in the future.