[css-logical-1] Expand note, adding warning for authors about problematic cases.

Author: fantasai

css-logical-1/Overview.bs

@@ -303,11 +303,16 @@ Flow-Relative Box Model Properties</h2>
   depends on the element’s computed values of
   'writing-mode', 'direction', and 'text-orientation'.
 
-  Note: Depending on the element’s own writing mode
+  Note: Depending on the element’s own <a>writing mode</a>
+  for mapping every <a>flow-relative</a> property
+  to its <a>physical</a> equivalent
   simplifies the cascading calculations
   and gives a straightforward model for authors to reason about.
   However, it is problematic in many cases,
   see for example <a href="https://www.w3.org/mid/20161108202634.GA7235@mail.internode.on.net">this discussion</a>.
+  Authors may need to use nested elements
+  to get the correct mapping behavior
+  when changing an element’s <a>writing mode</a> from its parent.
 
   A computed value that has flow-relative and physical properties as input
   is determined by applying the CSS cascade to declarations of both.