Add documentation for warning blocks in rustdoc book

src/doc/rustdoc/src/how-to-write-documentation.md

@@ -254,6 +254,18 @@ characters:
 
 So, no need to manually enter those Unicode characters!
 
+### Adding a warning block
+
+If you want to make a "warning" stand out in the documentation, you can wrap it like this:
+
+```md
+/// documentation
+///
+/// <div class="warning">A big warning!</div>
+///
+/// more documentation
+```
+
 [`backtrace`]: https://docs.rs/backtrace/0.3.50/backtrace/
 [commonmark markdown specification]: https://commonmark.org/
 [commonmark quick reference]: https://commonmark.org/help/