Improve docs on Spring + shared engine integration

When using Spring and a container managed engine you have to add "camunda-engine-spring" to the wabapp (it does NOT go into the shared library of the container). This should be mentioned explicitly in the docs (e.g. around http://docs.camunda.org/latest/guides/user-guide/#spring-framework-integration-expression-resolving-expression-resolving-with-the-shared-process-engine).