Explain double colon :: syntax for cast

Add a hint to invoke `cast` instead of using the double colon `::` syntax. I could not find an explanation how to render the `col::type` syntax and it took an excursion to the history of the Slack `#honeysql` channel to figure out what to do. This documents my findings. References: https://clojurians.slack.com/archives/C66EM8D5H/p1702579675520789?thread_ts=1702577803.253389&cid=C66EM8D5H