proc.review.role doesn't explain why roles exist

[thejayps]

Explaining the purpose of roles can help leaders to understand the flexibility available to them in review planning, eg that a role could be split among 2 people.

https://github.com/Ravenbrook/mps/blob/586336a7021c2276a3f798ad10e6e4ab5f438226/procedure/review.rst#52-review-planning