Update manual testing doc with syntax highlighting check.

doc/release/manual_testing.md

@@ -43,6 +43,10 @@ Use the [support document](https://docs.google.com/document/d/1cAHvbdFE6zR5WY-zh
 - Check if the links in the README when viewed as a [blob](https://dev.gitlab.org/samples/relative-links/blob/master/README.md) are correct
 - Select the "markdown" branch and check if all links point to the files within the markdown branch
 
+## Syntax highlighting
+- Visit/clone [language highlight repository](https://dev.gitlab.org/samples/languages-highlight)
+- Check for obvious errors in highlighting
+
 ## Upgrader
 - Upgrade from the previous release
 - Run the upgrader script in this release (it should not break)