Document how to confirm that `INSERT` worked with prepared statement

When using `execute()` I can parse the [command output for INSERT](https://www.postgresql.org/docs/current/sql-insert.html) to confirm a single row inserted like `INSERT 0 1`. However, `execute()` does not support the LRU for automatically prepared statements. Can you document how to confirm that the `INSERT` worked using a `fetch*()` method that will use the LRU? I do sometimes see `INSERT 0 0` results in some high-volume workloads (likely just duplicate data), and these do not raise `UniqueViolationError`, `QueryCanceledError`, or some `TimeoutError`.

Start by reviewing asyncpg's documented execute() and fetch*() methods alongside PostgreSQL's INSERT command documentation. Determine how a fetch-based call can confirm whether an INSERT affected a row while using the prepared-statement LRU, including the meaning of zero affected rows. Done means the documentation clearly explains the recommended method and expected result.

Written by the indexing model from the issue text.