[Improvement]: Mention that "Linking to code" isn't supported in markdown files (#54134)

Author: sophietheking

content/get-started/writing-on-github/working-with-advanced-formatting/creating-a-permanent-link-to-a-code-snippet.md

@@ -18,7 +18,7 @@ shortTitle: Permanent links to code
 
 ## Linking to code
 
-This type of permanent link will render as a code snippet only in the repository it originated in. In other repositories, the permalink code snippet will render as a URL.
+This type of permanent link will render as a code snippet only in the repository it originated in. In other repositories, the permalink code snippet will render as a URL. This does not work in Markdown files, only in comments.
 
 ![Screenshot of an issue comment. A code snippet has a header that lists the file name and line numbers, and a body that lists the code on those lines.](/assets/images/help/repository/rendered-code-snippet.png)