Explain how `on_schema_change` handles new or removed columns for an incremental model

### 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/docs/build/incremental-models#what-if-the-columns-of-my-incremental-model-change

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

@Fleid noted [here](https://github.com/dbt-labs/dbt-adapters/issues/601) that the following is a more complete and accurate description of how the value of `on_schema_change` behaves for incremental models when:
1. new columns are added, and
2. when columns are removed

|`on_schema_change`|In case of new columns...|In case of removed columns...|
|-|-|-|
|`ignore`|columns ignored|run failed|
|`fail`|run failed|run failed|
|`append_new_columns`|columns added|columns ignored|
|`sync_all_columns`|columns added|columns removed|

### Additional information

_No response_