Servlet parameter documentation for Spring + Flow

[OlliTietavainenVaadin]

There are some Vaadin-specific servlet parameters (heartBeatInterval, closeIdleSessions etc.) that need to be configured for different environments from time to time. The existence of those parameters is documented here: https://vaadin.com/docs/v10/flow/advanced/tutorial-application-lifecycle.html and https://vaadin.com/docs/v10/flow/spring/tutorial-spring-configuration.html mentions "There are a number of properties that you can use to configure your Flow application. See e.g. com.vaadin.server.DeploymentConfiguration and com.vaadin.server.Constants classes for property names.", but the documentation lacks proper usage examples.