docs(revealjs): document tabset fragments

docs/presentations/revealjs/index.qmd

@@ -398,7 +398,52 @@ Content for `Tab B`
 :::
 ```
 
-Note that one significant disadvantage to tabsets is that only the first tab will be visible when printing to PDF.
+When you print to PDF, only the active tab of a tabset shows.
+The active tab is the first tab, unless another tab has the `.active` class.
+To print all the tabs, use tabset fragments.
+
+### Tabset fragments
+
+{{< prerelease-callout 1.12 >}}
+
+Set the `tabset-fragments` option to step through the tabs of each tabset as fragments:
+
+``` yaml
+format:
+  revealjs:
+    tabset-fragments: true
+```
+
+Each step of the presentation shows the next tab.
+The fragments in a tab show one by one before the next tab.
+A `fragment-index` sets the order of the fragments in its own tab only.
+The tabset always starts on the first tab, and the `.active` class has no effect.
+
+To change one tabset only, add a class to that tabset:
+
+- `.tabset-fragments` turns on the steps for the tabset, when the option is off.
+- `.no-tabset-fragments` turns off the steps for the tabset, when the option is on.
+
+A tabset inside a tabset that does not step does not step either, whatever its class.
+
+``` markdown
+::: {.panel-tabset .tabset-fragments}
+
+### Tab A
+
+Content for `Tab A`
+
+### Tab B
+
+Content for `Tab B`
+
+:::
+```
+
+When you print to PDF, each tab of a tabset with fragments shows on a page:
+
+- With `pdf-separate-fragments: true`, each step gets its own page, as for other fragments.
+- With `pdf-separate-fragments: false`, each tab gets one page, and the slide numbers show as `2.1`, `2.2`, and so on.
 
 ## Slide Backgrounds