Add member_role_id to group create

What does this MR do and why?

This adds documentation to the POST endpoint of creating a member permission on a group or project. This is works on the api and is documented on the update/ PUT event.

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I'm working on a few other tickets (adding this feature to terraform), but you can see in this the log messages show a 201 on a POST to this endpoint.

terraform-provider-gitlab#6186 https://github.com/xanzy/go-gitlab/issues/1867