[css-writing-modes] Clarify that mappings are based on used direction (per existing definitions in #logical-directions)

Author: fantasai

css-writing-modes-3/Overview.bs

@@ -1674,7 +1674,8 @@ Line-relative Directions</h3>
 <h3 id="logical-to-physical">
 Abstract-to-Physical Mappings</h3>
 
-  <p>The following table summarizes the abstract-to-physical mappings:</p>
+  <p>The following table summarizes the abstract-to-physical mappings
+  (based on the <em>used</em> 'direction' and 'writing-mode'):
 
   <table class="complex data">
     <caption>Abstract-Physical Mapping</caption>

css-writing-modes-4/Overview.bs

@@ -1759,7 +1759,8 @@ Line-relative Directions</h3>
 <h3 id="logical-to-physical">
 Abstract-to-Physical Mappings</h3>
 
-  <p>The following table summarizes the abstract-to-physical mappings:</p>
+  <p>The following table summarizes the abstract-to-physical mappings
+  (based on the <em>used</em> 'direction' and 'writing-mode'):
 
   <table class="complex data">
     <caption>Abstract-Physical Mapping</caption>