:::notes div works with beamer, need to update documentation

[folofjc]

Explain the problem.
The pandoc manual here states that speaker notes work with powerpoint and reveal.js. However, pandoc also translates them into beamer, so should update the manual to show that they work in beamer.

For context, see this discussion.

For completeness, what pandoc does is convert the content of the div block into a \note{....} command in LaTeX beamer.

Pandoc version?
2.11.2 on Ubuntu 20.04 LTS