ConfigurableWebApplicationContext needed for contextClass parameter

Issue: SPR-17414

(cherry picked from commit 1c67ef4)

Author: jhoeller

src/docs/asciidoc/web/webmvc.adoc

@@ -457,8 +457,8 @@ initialization parameters ( `init-param` elements) to the Servlet declaration in
 | Parameter| Explanation
 
 | `contextClass`
-| Class that implements `WebApplicationContext`, which instantiates the context used by
-  this Servlet. By default, the `XmlWebApplicationContext` is used.
+| Class that implements `ConfigurableWebApplicationContext`, to be instantiated and
+  locally configured by this Servlet. By default, `XmlWebApplicationContext` is used.
 
 | `contextConfigLocation`
 | String that is passed to the context instance (specified by `contextClass`) to