Description

I was exploring the examples folder of this repository and came across the following lines:
https://github.com/slackapi/bolt/blob/522e70b381cf3d18a88b7ca271dcfb4f0ce1be9b/examples/hubot-example/script.js#L108-L113

Tested them in my app, they work great!

But, there are a couple of issues related to directMention() function.

1. It would be great to see more examples of directMention() in the official documentation. Currently, we have very little of the information about directionMention in "Listener middleware" section.
2. There are no corresponding typings. If I copy the example above to my TS project I'm getting compilation errors.

What type of issue is this? (place an x in one of the [ ])

• bug
• enhancement (feature request)
• question
• documentation related
• testing related
• discussion

Requirements (place an x in each of the [ ])

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue.