docs: Update strong documentation

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

docs/definitions.md

@@ -221,10 +221,12 @@ results in
 
 ```
 **bold text**
+__bold text__
 ```
 results in
 ```html
 <strong>bold text</strong>
+<strong>bold text</strong>
 ```
 
 ## emphasized