Update Slack integration prereqs (#4856)

## What are you changing in this pull request and why?

Update prereq in [Slack
notifications](https://docs.getdbt.com/docs/deploy/job-notifications#slack-notifications)
to include more info about channels

## Checklist
- [x] Review the [Content style
guide](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/content-style-guide.md)
so my content adheres to these guidelines.
- [x] For [docs
versioning](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#about-versioning),
review how to [version a whole
page](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#adding-a-new-version)
and [version a block of
content](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#versioning-blocks-of-content).

website/docs/docs/deploy/job-notifications.md

@@ -52,6 +52,7 @@ Any account admin can edit the Slack notifications but they'll be limited to con
 ### Prerequisites 
 - You must be an administrator of the Slack workspace. 
 - You must be an account admin to configure Slack notifications in dbt Cloud. For more details, refer to [Users and licenses](/docs/cloud/manage-access/seats-and-users).
+- Make sure the notification channel (where you want to receive alerts) is a public channel. The integration only supports public channels in the Slack workspace. 
 
 ### Set up the Slack integration