Suggest to specify `ref` when there are multiple 1:many edges declared for an ent

Right now the error says:

```
400 Bad Request: Error: Hit an error while evaluating your schema:
Uncaught Error: Edge "from" in table "users" has too many potential inverse edges in table "friend_requests": "from", "to"
    at defineEntSchema (../node_modules/convex-ents/src/schema.ts:63:10)
    at <anonymous> (../convex/schema.ts:6:2)
```