[css-position] position: fixed always establishes a stacking context

Clarifies that an element with 'position: fixed' always establishes a
new stacking context. (This is different from 'position: absolute',
where 'z-index' determines if the element establishes a stacking context
or not.)

Author: atanassov

css-position/Overview.bs

@@ -1976,8 +1976,8 @@ Layered presentation</h2>
 
     <dt><dfn>auto</dfn></dt>
     <dd>
-      The stack level of the generated box in the current stacking context is 0. The box
-      does not establish a new stacking context unless it is the root element.
+      The stack level of the generated box in the current stacking context is 0. 
+      If the box has 'position: fixed' or if it is the root, it also establishes a new stacking context. 
   </dl>
 
   In the following example, the stack levels of the boxes (named with their "id"