add grouping to docs

internal/packets/PARSER.md

@@ -77,6 +77,18 @@ Fast XML-like parser for binary packet structures with versioning and conditiona
 **Array Context:**
 - `item_type_%i!=0` - `%i` substitutes current array index
 
+## Groups
+
+Organize related fields with automatic prefixing:
+
+```xml
+<group name="appearance">
+	<color name="skin_color,hair_color,eye_color">
+	<str16 name="face_file,hair_file">
+</group>
+<!-- Creates: appearance_skin_color, appearance_hair_color, etc. -->
+```
+
 ## Arrays
 
 ```xml