Improve the contribution and MR review guide

## What does this MR do?

This merge request improves the contributing and MR review guides following @stanhu's reply on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6096#note_16537211.

## Why was this MR needed?

To clearly state that MR reviews can take multiple iterations but that as reviewers we should do our best to minimize the number of iterations and take over any MR that is set as "Merge when build succeeds" at some point.

See merge request !6739