Document unpartitioned Hive `insert_overwrite` behavior for Athena

### Contributions

- [x] I have read the contribution docs, and understand what's expected of me.

### Link to the page on docs.getdbt.com requiring updates

https://docs.getdbt.com/reference/resource-configs/athena-configs

### What part(s) of the page would you like to see updated?

The "Incremental table models" section currently documents `insert_overwrite` as always falling back to `append` when no partitions are defined.

dbt-labs/dbt-adapters#2082 proposes distinguishing an omitted strategy from an explicitly configured `insert_overwrite` for unpartitioned Hive models:

- When the strategy is omitted, preserve the existing `append` fallback.
- When `insert_overwrite` is explicit and dbt creates a new S3 location per run, which is the default, replace the complete table.
- When the model reuses an S3 path through `external_location` or `s3_data_naming`, preserve `append` and emit an informational message.
- Preserve existing behavior for partitioned Hive models and all other strategies.

The Athena configuration page should document this distinction when the adapter change is released.

### Suggested replacement copy

Replace the current `insert_overwrite` bullet with:

- `insert_overwrite` (default for Hive): For partitioned models, dbt deletes overlapping partitions from the destination table and inserts the new records. For unpartitioned models, an explicitly configured `incremental_strategy='insert_overwrite'` replaces the complete table when dbt creates a new S3 location per run (`s3_data_naming` contains `unique`, which is the default, and `external_location` is not configured). If the strategy is omitted, dbt preserves the historical `append` fallback. Models that explicitly request `insert_overwrite` but reuse the same S3 path also preserve `append` and emit an informational message.

### Additional information

Feature issue: https://github.com/dbt-labs/dbt-adapters/issues/2081

Implementation PR: https://github.com/dbt-labs/dbt-adapters/pull/2082

The dbt-athena README is also updated in the implementation PR.