Document :HTML_CONTAINER_CLASS: property

* doc/org.texi (CSS support): Document :HTML_CONTAINER_CLASS: property

doc/org.texi

@@ -9673,6 +9673,11 @@ For longer style definitions, you can use several such lines.  You could also
 directly write a @code{<style>} @code{</style>} section in this way, without
 referring to an external file.
 
+In order to add styles to a subtree, use the @code{:HTML_CONTAINER_CLASS:}
+property to assign a class to the tree.  In order to specify CSS styles for a
+particular headline, you can use the id specified in a @code{:CUSTOM_ID:}
+property.
+
 @c FIXME: More about header and footer styles
 @c FIXME: Talk about links and targets.