docs: add example combining when/unless for revealjs/html (#910)

Author: mcanouil

docs/authoring/conditional.qmd

@@ -45,6 +45,22 @@ Will not appear in PDF.
 
 Then `when-format` and `unless-format` attributes match the current Pandoc output format with some additional intelligence to alias related formats (e.g. html, html4, and html5). Details are provided below in [Format Matching](#format-matching)
 
+`when-format` and `unless-format` can also be combined to create more complex conditions:
+
+``` markdown
+::: {.content-visible when-format="html" unless-format="revealjs"}
+
+Will only appear in HTML and not in Reveal.js, but actually it appears.
+
+:::
+
+::: {.content-visible when-format="revealjs"}
+
+Will only appear in Reveal.js and not in HTML or other formats.
+
+:::
+```
+
 ## .content-hidden
 
 To prevent content from being displayed when rendering to a given format, use the `.content-hidden` class. For example, here we mark content as hidden in HTML: