[DOCS]: Better docs for `onConflictDoNothing`

### Describe what you want

Currently, this is the only comment about this method:

> `onConflictDoNothing` will cancel the insert if there’s a conflict

Something I see repeatedly in both Discord help channels and GH issues is users bringing up the fact that if there's a conflict while using this method, no row is returned. This behavior is intended by the database engine; however, Drizzle should inform the developer about this behavior in the documentation. This will avoid more duplicate issues here on Github and Discord.