Add support for gracefully shutting down the web server

This commit adds support for gracefully shutting down the embedded
web server. When a grace period is configured
(server.shutdown.grace-period), upon shutdown, the web server will no
longer permit new requests and will wait for up to the grace period
for active requests to complete.

Closes gh-4657

spring-boot-project/spring-boot-docs/src/docs/asciidoc/spring-boot-features.adoc

@@ -2960,6 +2960,26 @@ You can learn more about the resource configuration on the client side in the <<
 
 
 
+[[boot-features-graceful-shutdown]]
+== Graceful shutdown
+Graceful shutdown is supported with all four embedded web servers (Jetty, Reactor Netty, Tomcat, and Undertow) and with both reactive and Servlet-based web applications.
+When enabled, shutdown of the application will include a grace period of configurable duration.
+During this grace period, existing requests will be allowed to complete but no new requests will be permitted.
+The exact way in which new requests are not permitted varies depending on the web server that is being used.
+Jetty, Reactor Netty, and Tomcat will stop accepting requests at the network layer.
+Undertow will accept requests but respond immediately with a service unavailable (503) response.
+
+Graceful shutdown occurs as one of the first steps during application close processing and before any beans have been destroyed.
+This ensures that the beans are available for use by any processing that occurs while in-flight requests are being allowed to complete.
+To enable graceful shutdown, configure the configprop:server.shutdown.grace-period[] property, as shown in the following example:
+
+[source,properties,indent=0,configprops]
+----
+server.shutdown.grace-period=30s
+----
+
+
+
 [[boot-features-rsocket]]
 == RSocket
 https://rsocket.io[RSocket] is a binary protocol for use on byte stream transports.