Explain translation of `sslmode` to `sslmode` + `ssl` for `redshift_connector`

### 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/warehouse-setups/redshift-setup

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

See here for useful conversion tables:
https://github.com/dbt-labs/dbt-redshift/pull/439#issue-1706920612

### Additional information

Starting in dbt-redshift 1.5, the Python driver switched from `psycopg2` to `redshift_connector`. The latter has a `ssl` parameter (boolean) that the former did not have.

Rather than introduce a new config item for profiles, we partnered with the Amazon Redshift team to create conversion logic. We want to explain that conversion logic so that users can understand what is happening behind the scenes.