[css-inline] add note on properties applicable to initial-letter

Author: dauwhe

css-inline/Overview.bs

@@ -413,6 +413,8 @@ The following properties apply to an <a>initial letter</a>:
 	<li>all margin, border and padding properties  (see [[CSS21]] and [[CSS3BG]])
 </ul>
 
+Note: Which properties apply to <a>initial letter</a> depend on whether it is an inline box or an <a>initial letter</a> box, as <a>initial letter</a> is a display type.  
+
 Issue: Add 'width'/'height' as requested by Tantek... need to integrate with model.
 
 <h4 id="initial-letter-model">