[css-content] add issue to define accessibility of generated text

Author: dauwhe

css-content/Overview.bs

@@ -128,6 +128,8 @@ Issue: should the contents keyword be replaced with ''content()''
 	<p id="replacedContent">If the computed value of the part of the 'content' property that ends up being used is a single URI, then the element or pseudo-element is a replaced element. The box model defines different rules for the layout of replaced elements than normal elements. Replaced elements do not have '::before' and '::after' pseudo-elements; the 'content' property in the case of replaced content replaces the entire contents of the element's box.
 
 
+    Issue: State that generated content should be searchable, selectable, and available to assistive technologies. 
+
 
 
 <h2 id="content-functions">