TNL-328 Cohort/server display auto

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

In the doc, I've been describing the choice between auto and manual (and hybrid) as the "assignment strategy". I also call the groups "auto cohort group" and "manual cohort group". So maybe the first sentence could read:
The assignment strategy of this cohort group is:
or
This is a (manual/auto) cohort group. (with the variable in parens)