style-guide: add emphasis section (#11971)

Co-authored-by: Juri Dispan <juri.dispan@posteo.net>
Co-authored-by: Sebastiaan Speck <12570668+sebastiaanspeck@users.noreply.github.com>

contributing-guides/style-guide.md

@@ -330,6 +330,10 @@ For instance, instead of `Listing all files:`, `List all files:` can be used as
  `ls`
 ```
 
+## Emphasis
+
+Do not use *italics*, **boldface** or any other text styling in the pages. These are reserved for client emphasis of placeholders.
+
 ## Serial Comma
 
 - When declaring a list of 3 or more items,