jss-syntax documentation for high specificity

Document more uses for `&` when dealing with high specificity selectors

Author: Steffan-Ennis

docs/jss-syntax.md

@@ -338,6 +338,31 @@ Compiles to:
 }
 ```
 
+## Specific selectors
+
+When extending third party libraries with high secificity selector it's often necessary to have also have a high specificity.
+
+```javascript
+
+const styles = {
+  button: {
+    '.button &': {
+      color: 'red'
+    }
+  }
+}    
+
+```
+
+Compiles to:
+
+```css
+.button .button-0 {
+  color: 'red'
+}
+```
+
+
 ## Working with colors
 
 You can use any color conversion tool, e.g. [this one](https://yarnpkg.com/en/package/color).