If you upload the flow to the automate community, you could use simple html like this:
<strong>this is some bold text</strong>
<em>and this is some emphasized text</em>
It's maybe a little bit confusing how it works:
1. If you set the description on your flow, it doesn't get formatted
2. Once you upload that flow to the automate community, the description in the automate community will be formatted.
3. When people download that flow, the text get normalized and it just shows as if you didn't apply any formatting
The easier and better working option would be something like:
(This is a flow to format your text, this makes the text formatted everywhere and not just in the description of the automate community)