[css-writing-modes] Markup fix: qualify start/end to help disambiguate cross-references.

Author: fantasai

css-writing-modes-3/Overview.bs

@@ -1578,7 +1578,7 @@ Flow-relative Directions</h3>
   </dl>
 
   <p>Where contextually unambiguous or encompassing both meanings,
-    the terms <dfn export>start</dfn> and <dfn export>end</dfn>
+    the terms <dfn export for=CSS>start</dfn> and <dfn export for=CSS>end</dfn>
     are used in place of <a>block-start</a>/<a>inline-start</a>
     and <a>block-end</a>/<a>inline-end</a>, respectively.
 

css-writing-modes-4/Overview.bs

@@ -1650,7 +1650,7 @@ Flow-relative Directions</h3>
   </dl>
 
   <p>Where contextually unambiguous or encompassing both meanings,
-    the terms <dfn export>start</dfn> and <dfn export>end</dfn>
+    the terms <dfn export for=CSS>start</dfn> and <dfn export for=CSS>end</dfn>
     are used in place of <a>block-start</a>/<a>inline-start</a>
     and <a>block-end</a>/<a>inline-end</a>, respectively.