Force documentation of nested group

The `strategy` group is not detected in the meta-data because the
Strategy inner class is not defined at the "right" level in the
hierarchy.

For now, `@NestedConfigurationProperty` was added to workaround the
issue. Once gh-3454 is solved, we should remove them.

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ResourceProperties.java

@@ -22,6 +22,7 @@ import java.util.Collections;
 import java.util.List;
 
 import org.springframework.boot.context.properties.ConfigurationProperties;
+import org.springframework.boot.context.properties.NestedConfigurationProperty;
 import org.springframework.context.ResourceLoaderAware;
 import org.springframework.core.io.ClassPathResource;
 import org.springframework.core.io.Resource;
@@ -168,6 +169,7 @@ public class ResourceProperties implements ResourceLoaderAware {
 		 */
 		private boolean htmlApplicationCache = false;
 
+		@NestedConfigurationProperty
 		private final Strategy strategy = new Strategy();
 
 		public Boolean getEnabled() {
@@ -205,8 +207,10 @@ public class ResourceProperties implements ResourceLoaderAware {
 	 */
 	public static class Strategy {
 
+		@NestedConfigurationProperty
 		private final Fixed fixed = new Fixed();
 
+		@NestedConfigurationProperty
 		private final Content content = new Content();
 
 		public Fixed getFixed() {