TNL-328 Cohort/server display auto

[TNL-180](https://openedx.atlassian.net/browse/TNL-180)

@lamagnifica @explorerleslie Any thoughts on updating this text?
It would read:
``` 
This cohort group's current management set up: 
Students are added to this group only when you provide their email addresses or usernames on this page.
```
OR
``` 
This cohort group's current management set up: 
Students are added to this group automatically.
```