Docs: Clarify merge request Assignee vs Reviewer vs Approver

Problem to solve

What is the purpose of Assignee vs Reviewer vs Approver? Is an Approver also inherently a Reviewer?

• Assignee currently per https://docs.gitlab.com/ee/user/project/merge_requests/getting_started.html#assignee:

  Choose an assignee to designate someone as the person responsible for the first review of the merge request

• Reviewer currently per https://docs.gitlab.com/ee/user/project/merge_requests/getting_started.html#reviewer

  The Merge Request Reviewers feature enables you to request a review of your work, and see the status of the review. Reviewers help distinguish the roles of the users involved in the merge request. In comparison to an Assignee, who is directly responsible for creating or merging a merge request, a Reviewer is a team member who may only be involved in one aspect of the merge request, such as a peer review.

• Can't find basic definition of Approver but some details mixed in https://docs.gitlab.com/ee/user/project/merge_requests/merge_request_approvals.html

These sections of docs seem to partially contradict one another. Additionally which can be assigned and how many in which get edition? Appears Assignee is documented but not Reviewer regarding if it supports multiple or not. It appears Approvers are not assigned but rather implied by approving MR?

Further details

Proposal

Who can address the issue

@aqualls @sselhorn per https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments

Other links/references

May also help address confusion in #292938 (closed).