Add comment in all PrimaryOnlyServices that don't utilize 'existingInstances'

SERVER-60752 passed a vector of 'existingInstances' on a new PrimaryOnlyService instance's construction, to allow each service to first check if there are any conflicting instances.

However, pre-existing services have already implemented conflict detection their own ways, so don't utilize 'existingInstances.'

It will help future readers to add a comment to this effect in the pre-existing services.