Add explanation on conditional code execution

docs/authoring/conditional.qmd

@@ -25,6 +25,23 @@ You can also set conditions on non-executable code blocks:
 ```
 ````
 
+::: {.callout-tip title="Conditional Exectution with Knitr" collapse="true"}
+
+When using the `knitr` engine, if you want to have an **executable** code chunk being executed/evaluated only when the output is being rendered to HTML or LaTeX you can use the `is_html_output()` and `is_latex_output()` respectively (see [this reply to quarto-cli issue 11724](https://github.com/quarto-dev/quarto-cli/issues/11724#issuecomment-2557635192)).
+
+```` markdown
+::: {.content-visible when-format="html"}
+
+```{{r}}
+#| eval: !expr knitr::is_html_output()
+# code is shown AND evaluated only in HTML
+2+2
+```
+
+:::
+````
+:::
+
 To apply a condition only to a part of a paragraph, use a span (`[]{}`):
 
 ``` markdown
@@ -112,4 +129,4 @@ path:
 ````
 
 This feature is often useful alongside [project profiles](/docs/projects/profiles.qmd).
-Different profiles can set different metadata values, and so can control the metadata used in conditional content.
+Different profiles can set different metadata values, and so can control the metadata used in conditional content.