note cases where em tag won't work (#30707)

Co-authored-by: Ethan Palm <56270045+ethanpalm@users.noreply.github.com>

contributing/content-markup-reference.md

@@ -72,7 +72,7 @@ To render syntax highlighting in command line instructions, we use triple backti
 
 This syntax highlighting renders light text on a dark background, and should be reserved for command line instructions.
 
-Within the command-line syntax, you can also use the `<em>` helper tag to indicate content that varies for each user, such as a user or repository name.
+Within the command-line syntax, you can also use the `<em>` helper tag to indicate content that varies for each user, such as a user or repository name. **Note**: If you specify a language other than `sh` or `shell` or you use `{:copy}`, the `<em>` tag will not render correctly.
 
 **Copy-able code blocks**
 
@@ -82,6 +82,9 @@ You can also add a header that includes the name of the language and a button to
     const copyMe = true
     ```
 
+**Note**: The `<em>` tag will not render correctly with the `copy` button.
+
+
 ## Octicons
 
 Octicons are icons used across GitHub’s interface. We reference Octicons when documenting the user interface. Find the name of the Octicon on the [Octicons site](https://primer.style/octicons). For accessibility purposes, use [the `aria-label` option](https://primer.style/octicons/packages/javascript#aria-label) to describe the Octicon.