Document need for ServerEndpointExporter and show its use in a sample

Traditionally, a @ServerEndpoint-annotated bean is found by a servlet
container initialiser, however Boot does not run servlet container
initialisers when an embedded container is being used. To be able to use
@ServerEndpoint in a Boot app that uses embedded Tomcat a
ServerEndpointExporter bean must be declared.

This commit updates the documentation to describe this requirement and
also updates the WebSockets sample to illustrate the use of
ServerEndpointExporter. The version of Spring Framework has been updated
to 4.0.8.BUILD-SNAPSHOT. This picks up the fix for SPR-12340.

Closes gh-1722