Update docs for slack_config

This commit updates the docs for slack_config with usage for both
Incoming webhooks and Bot tokens.

Signed-off-by: George Robinson <george.robinson@grafana.com>

docs/configuration.md

@@ -951,9 +951,13 @@ token_file: <filepath>
 
 ### `<slack_config>`
 
-Slack notifications are sent via [Slack
-webhooks](https://api.slack.com/messaging/webhooks). The notification contains
-an [attachment](https://api.slack.com/messaging/composing/layouts#attachments).
+Slack notifications can be sent via [Incoming webhooks](https://api.slack.com/messaging/webhooks) or [Bot tokens](https://api.slack.com/authentication/token-types).
+
+If using an incoming webhook then `api_url` must be set to the URL of the incoming webhook, or written to the file referenced in `api_url_file`.
+
+If using Bot tokens then `api_url` must be set to [`https://slack.com/api/chat.postMessage`](https://api.slack.com/methods/chat.postMessage), the bot token must be set as the authorization credentials in `http_config`, and `channel` must contain either the name of the channel or Channel ID to send notifications to. If using the name of the channel the # is optional.
+
+The notification contains an [attachment](https://api.slack.com/messaging/composing/layouts#attachments).
 
 ```yaml
 # Whether to notify about resolved alerts.