[css-content] change def of content property to have image | url

Author: dauwhe

css-content/Overview.bs

@@ -80,7 +80,7 @@ figure:not([alt]) { content: attr(href, url), contents; }
 
 <pre class='propdef'>
     Name: content
-    Value: [ [ <<uri>> ] ',' ]* [ normal | none | <<content-list>> ] [/ <<string>> ]?
+    Value: [ [ <<image>> | <<url>> ] ',' ]* [ normal | none | <<content-list>> ] [/ <<string>> ]?
     Initial: normal
     Applies To: all elements, ''::marker'',  and page margin boxes.
     Inherited: no
@@ -139,8 +139,6 @@ Content Values and Functions</h2>
     including images, strings, the values of counters, and the text value of elements. 
     In this section we enumerate the possibilities. 
 
-	Issue: write better introduction to this section
-
 
 <h3 id="content-string">
 String</h3>