[css-content] add subsection on accessibility, rewrite issue

Author: dauwhe

css-content/Overview.bs

@@ -128,10 +128,12 @@ 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.
 
+<h3 id="accessibility-of-generated-content">
+Accessibility of Generated Content</h3>
 
-    Issue: State that generated content should be searchable, selectable, and available to assistive technologies. 
-
+   Generated content should be searchable, selectable, and available to assistive technologies. 
 
+   Issue: Start work on an AAM for CSS. 
 
    Generated content often needs alternative text. 
    If the value of the content property is a string,