Add documentation about using PrivateRelation

[alexdunae]

Motivation

I was migrating from sorbet-rails and couldn't find a migration path for RelationType. I searched the Slack channel and came up with this suggestion. I figured it would be helpful to others.

Implementation

n/a

Tests

just docs

[alexdunae]

I've just signed the CLA

[fazo96]

This looks like it could be a solution for #1140