I've long since forgotten where I saw it, but it was something like "if you're writing an email and it's more than two paragraphs, publish it somewhere: a wiki, Twitter thread, blog, etc.
Putting an explanation for how to do something in Slack means it will almost certainly get lost immediately. Even if you're decent at searching, maybe others aren't, and you can't search for something if you don't know it's there!
Instead, putting it in backstage.io documentation (if it's work related), or a blog or digital garden if not, means that not only can you easily share it in the future, but so can others.