Add descriptions for GroupMemberRelationEnum

What does this MR do?

This MR adds descriptions for GroupMemberRelationEnum. These descriptions are derived from https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/finders/group_members_finder.rb#L38 and lib/gitlab/object_hierarchy.rb. This is part of the effort to update GraphQL enum type descriptions as described in #299489 (closed). This MR follows the same structure as the change in !57876 (merged).

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Does this MR meet the acceptance criteria?

Conformity

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  • [-] I have included a changelog entry.
  • I have not included a changelog entry because this is a docs only change.
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• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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Security

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Related to #299489 (closed)