docs: clarify additionalLanguages with custom language definitions. (#…

…8307)

Co-authored-by: Joshua Chen <sidachen2003@gmail.com>
Co-authored-by: sebastienlorber <lorber.sebastien@gmail.com>

website/docs/guides/markdown-features/markdown-features-code-blocks.mdx

@@ -136,6 +136,8 @@ const prismIncludeLanguages = (Prism) => {
 
 You can refer to [Prism's official language definitions](https://github.com/PrismJS/prism/tree/master/components) when you are writing your own language definitions.
 
+When adding a custom language definition, you do not need to add the language to the `additionalLanguages` config array, since Docusaurus only looks up the `additionalLanguages` strings in languages that Prism provides. Adding the language import in `prism-include-languages.js` is sufficient.
+
 ## Line highlighting {#line-highlighting}
 
 ### Highlighting with comments {#highlighting-with-comments}