add description to "Default channel for Slack notifications" UI dropd…

…own (#2051)

# What this PR does
<img width="1005" alt="Screenshot 2023-05-30 at 07 53 20"
src="https://github.com/grafana/oncall/assets/9406895/a3533104-5773-4ce4-b717-74feeca1f0f0">

## Checklist

- [ ] Unit, integration, and e2e (if applicable) tests updated (N/A)
- [ ] Documentation added (or `pr:no public docs` PR label added if not
required) (N/A)
- [x] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)

src/pages/settings/tabs/ChatOps/tabs/SlackSettings/SlackSettings.tsx

@@ -109,7 +109,10 @@ class SlackSettings extends Component<SlackProps, SlackState> {
                   <Text>{store.teamStore.currentTeam.slack_team_identity?.cached_name}</Text>
                 </div>
               </Field>
-              <Field label="Default channel for Slack notifications">
+              <Field
+                label="Default channel for Slack notifications"
+                description="The selected channel will be used as a fallback in the event that a schedule or integration does not have a configured channel"
+              >
                 <WithPermissionControlTooltip userAction={UserActions.ChatOpsUpdateSettings}>
                   <GSelect
                     showSearch